45. EG: 
BUL93 


1 62-607-lm-899 


BULLETIN 

OF  THE 

UNIVERSITY  OF  TEXAS 

NUMBER  93 

SCIENTIFIC  SERIES  NO.  11  APRIL  15,  1907 


A Sketch  of  the  Geology  of  the  Chisos 
Country,  Brewster  County,  Texas. 


By 


J.  A.  UDDEN,  Ph.  D.,  F.  G.  S.  A., 

Professor  of  Geology,  Augustana  College;  formerly  Assistant  Geologist,  University  of 
Texas  Mineral  Survey. 


Entered  at  the  Postoffice  in  Austin , Texas , as  second  class  mail  matter 
under  Act  of  Congress , passed  July  16,  1894 


AUSTIN.  TEXAS 


Cultivated  mind  is  the  guardian  genius  of  de- 
mocracy. . . . It  is  the  only  dictator  that  free- 
men acknowledge  and  the  only  security  that 
freemen  desire. 


President  Mirabeau  B.  Lamar. 


EDITOR'S  NOTE. 

This  Bulletin,  prepared  for  the  University  of  Texas  Mineral  Survey, 
under  the  direction  of  Dr.  William  B.  Phillips,  was  ready  for  publication 
in  the  spring  of  1905,  but,  owing  to  the  discontinuance  of  the  Survey, 
could  not  be  published  at  that  time.  Its  high  practical  and  scientific  value 
leads  to  its  issue  now  as  a Bulletin  of  the  University  of  Texas.  Unfortu- 
nately, both  the  drawings  and  cuts  originally  prepared  to  illustrate  it 
have  been  lost,  and  it  has  not  been  practicable  to  reproduce  them. 


CONTENTS 


PAGE. 

Introductory  statement  9 

A.  General  topography  9 

1.  Differential  erosion  10 

2.  The  graded  plains 10 

(1)  The  making  of  the  graded  plains 10 

(2)  Controlling  factors  11 

(3)  Maintenance  of  the  eroded  plain 13 

3.  The  high  reliefs 14 

B.  History  of  the  drainage 15 

C.  Stratigraphy 16 

1.  The  general  section 17 

(1)  The  Palaeozoic  18 

(2)  The  Ordovician  18 

(3)  The  Upper  Carboniferous 20 

(4)  Hocks  of  the  Cretaceous  age 21 

The  Lower  Cretaceous  limestones 21 

At  Persimmon  gap  22 

On  the  Altuda  mountain 22 

On  the  Cienega  mountain 23 

In  Mesa  de  Anguila 23 

On  Christmas  mountain 24 

On  Mariscal  mountain 24 

In  the  Carmen  range 25 

Characteristic  horizons  26 

Chemical  composition  27 

Characteristic  weathering  28 

The  Upper  Cretaceous  series 29 

The  Boquillas  flags 29 

Chemical  composition  29 

Clastic  texture  30 

Field  appearances  30 

Resistance  to  erosion 31 

Minor  characteristics  32 

Fossils „ 32 

Correlation  33 

The  Terlingua  beds  33 

Distribution  34 

Cottonwood  creek  section 34 

Section  south  of  Cuesta  Blanca 35 

Other  sections  36 

Topographic  relations  38 


6 


Contents. 


PAGE. 

Organic  remains  38 

Chemical  composition  39 

Correlations  40 

The  Rattlesnake  beds 41 

The  zone  of  change 41 

Thickness  . . 42 

Sandstones 42 

Finer  sediments  45 

Original  calcareous  deposits 45 

Special  sections  45 

Section  near  Chisos  pen 46 

Section  north  of  Chisos  pen 47 

Section  south  of  Chisos  pen 47 

Fossil  wood  48 

Vertebrate  remains  49 

Tuffs 49 

Topographic  features  51 

Fossils  51 

Correlation 54 

The  Tornillo  clays 54 

Characteristic  coloring  54 

Mechanical  composition  55 

Chemical  composition  56 

Concretions 57 

Weathering  and  creeping 57 

Geographical  distribution  58 

Physiographic  character  59 

Thickness  . 59 

Fossils 60 

The  Chisos  beds 60 

The  typical  rock 61 

Sandstones 62 

Clays 63 

Conglomerates 63 

Descriptions  of  exposures 64 

Limits  and  thickness 65 

Topography 65 

Life  and  age 66 

The  Crown  conglomerate 66 

The  Burro  gravels  and  tuffs 67 

The  Dugout  clays  and  gravels 68 

Recent  alluvium  and  land  drift 69 

(5)  The  igneous  rocks 70 

The  Altuda  granite 70 


Contents. 


( 

PAGE. 

Laccoliths  70 

Sills  72 

Dikes  73 

Plugs 74 

Surface  flows  74 

D.  Geological  structure  75 

E.  The  Caballos  ridges  76 

F.  Overlap  of  the  Cretaceous 78 

G.  The  Rocky  Mountain  uplift 79 

H.  The  great  Terlingua  fault 80 

I.  The  sunken  block 80 

1.  Faults 81 

2.  Folds  83 

3.  Christmas  mountain  fold 85 

4.  Mariscal  mountain  fold 85 

5.  Chisos  mountain  fold 86 

J.  Summary  of  geologic  history 87 

K.  Mineral  resources  89 

1.  Quicksilver  89 

2.  Coal  94 

3.  Silver  99 

4.  Phosphate  100 

5.  Brick  materials  100 


Digitized  by  the  Internet  Archive 
in  2017  with  funding  from 

University  of  Illinois  Urbana-Champaign  Alternates 


https://archive.org/details/sketchofgeologyoOOudde 


A SKETCH  OF  THE  GEOLOGY  OF  THE 
CHISOS  COUNTRY. 


In  studying  the  geology  of  the  Chisos  country,  one  must  con- 
stantly bear  in  mind  that  the  terranes  which  we  now  find  exposed 
at  the  surface  have  been  laid  bare  from  under  a covering  of  over- 
lying  rocks  measuring  from  two  to  ten  thousand  feet  in  depth. 
This  great  thickness  of  vanished  strata  has  been  removed  during  a 
long  lapse  of  time,  which  commenced  with  the  closing  stage  of  the 
Cretaceous  age.  We  should  also  remember  that  during  the  time 
this  work  of  disintegration  and  removal  was  going  on,  and  even 
before  it  began,  the  whole  ground  has  been  subjected  to  forces 
which  have  thrown  the  strata  into  folds  and  flexures  and  broken 
them  by  great  faults  into  blocks  that  are  extensively  tilted. 

As  a circumstance  which  has  contributed  to  the  effectiveness  of 
erosion  in  this  region,  we  must  regard  the  fact  that  it  lies  on  the 
border  of  the  great  Cordilleran  plateau,  where  the  drainage  has 
had  more  or  less  direct  outlets  to  the  region  of  the  Plains  on  the 
east.  The  descent  of  the  streams  has  been  relatively  rapid  and 
transportation  correspondingly  easy.  For  the  Santiago  and  Car- 
men ranges,  which  make  the  east  boundary  of  the  Chisos  country, 
must  be  regarded  as  the  southernmost  extension  of  the  Rocky 
Mountains  in  the  United  States.  To  the  west  of  these  ranges  we 
find  that  the  geological  structure  is  more  like  that  of  the  Great 
Basin  and  to  the  east,  though  the  superficial  aspect  is  somewhat 
mountain-like,  the  structure  is  that  of  the  bordering  high  plains. 

A.  GENERAL  TOPOGRAPHY. 

The  Chisos  mountains  constitute  the  highest  relief  in  a belt 
of  somewhat  interrupted  and  broken  plains,  which  extend  north- 
westward to  and  beyond  Marfa.  This  belt  has  suffered  a greater 
reduction  from  the  erosive  forces  than  the  mountains  around  it, 
owing  to  the  weak  resistance  of  the  clays  and  marls,  which  mostly 
constitute  the  bedrock  within  its  limits.  It  may  be  regarded  as  a 
narrow  extension  of  the  Howard  bolson*  described  by  Prof.  R.  T. 

^Physical  Geography  of  the  Texas  Region,  Topographic  Atlas  of  the. 
United  States,  United  States  Geological  Survey,  1900,  p.  9. 


10 


Bulletin  of  The  University  of  Texas. 


Hill.  To  the  east  and  to  the  west  of  this  plain  more  indurated 
rocks  are  now  exposed  and  rise  in  ridges  and  table-lands.  Toward 
the  Rio  Grande  the  plains  gradually  descend  to  an  elevation  of  only 
about  2000  feet  above  the  sea  level.  From  the  middle  of  this  de- 
scending plain  the  Chisos  mountains  rise  in  a low,  wide,  and  dis- 
sected dome,  with  long  spurs  to  the  south. 

1.  Differential  Erosion. 

The  materials  subjected  to  the  work  of  the  destructive  forces 
during  a long  time  in  the  past  in  this  region  consist  for  the  most 
part  of  soft  sediments,  such  as  clay,  deep-sea  ooze,  and  sand.  With 
these  are  extensive  beds  of  limestone  and  frequent  and  variously 
shaped  masses  of  igneous  rock  injected  into  the  sediments  and  in 
places  poured  out  over  them.  Almost  all  of  the  pronounced  relief 
Ave  meet  with  here,  such  as  hills,  ridges,  mountains,  and  table-lands, 
are  due  to  the  greater  resistance  of  the  harder  materials.  These 
have  been  laid  bare  by  the  removal  of  the  more  readily  yielding 
clays  and  marls,  by  which  they  were  originally  covered  or  sur- 
rounded. All  the  more  enduring  rocks  now  rise  above  the  general 
surface  of  the  land.  In  places  where  the  indurated  rocks  are  ab- 
sent, the  ground  has  been  cut  down  to  almost  perfect  plains.  The 
relief  of  these  plains  is  confined  to  a very  gentle  general  slope 
towards  the  drainage  channels,  low  scarps  caused  by  transient  ad- 
justments to  new  erosion  planes,  and  long,  narrow  and  shallow 
gullies  of  the  surface  drainage. 

2.  The  Graded  Plains. 

The  controlling  topographic  feature  of  the  country  around  the 
Chisos  mountains  consists  of  the  plains  just  referred  to.  It  is 
estimated  that  they  make  up  from  two-thirds  to  three-fourths  of 
the  entire  area  of  the  country.  Their  general  slope  averages  a 
hundred  feet  to  the  mile,  and  varies  from  seventy-five  feet,  where 
it  is  the  most  gentle,  to  two  hundred  feet,  where  it  is  most  steep. 
Only  in  few  places,  on  some  low  divides,  it  is  nearly  horizontal. 

(1)  Making  of  the  Graded  Plains. 

The  origin  of  the  high  plains  in  the  southwest,  the  so-called 
bolson  plains,  has  been  ascribed  to  different  causes  by  different 


The  Geology  of  the  Ghisos  Country. 


11 


writers.  E.  T.  Hill  regards  them  as  constructional  detritus  plains 
caused  by  filling  of  structural  valleys.* 

Keyes  considers  that  the  bolsons  are  a part  of  an  upraised  pene- 
plain, practically  untouched  by  stream  action.**  It  does  not  ap- 
pear that  either  of  these  explanations  apply  well  to  the  plains  of 
the  Chisos  country.  The  drift  filling  is  everywhere  of  small  depth, 
probably  nowhere  amounting  to  as  much  as  two  hundred  feet.  Nor 
is  there  any  general  level  indicating  a peneplain.  If  the  graded 
plains  here  have  once  been  a part  of  such  a plain,  this  has  later 
suffered  extensive  destruction  by  recent  erosion.  The  present  re- 
lief of  the  plains  in  this  region  must  be  regarded  as  expressing  an 
equilibrium  between  recent  erosion  and  transportation  on  the  one 
hand  and  the  resistance  of  the  materials  subjected  to  these  forces 
on  the  other.  It  is  the  direct  result  of  general  erosion  now  in 
progress.  This  is  always  accompanied  by  local  accumulations  of 
land  drift,  owing  to  local  variations  in  the  factors  which  determine 
the  effectiveness  of  the  agents  at  work,  and  thus  the  plains  are  here 
the  result  of  constructive  work  as  well  as  destructive. 

It  will  be  perceived  that  the  grade  of  plains  whose  existence 
depends  on  such  nicely  balanced  conditions  will  be  subject  to  some 
changes.  Whenever  weathering  is  progressing  too  slowly  to  fur- 
nish as  much  materials  as  the  transporting  agencies  are  able  to  re- 
move, the  grade  of  the  plain  will,  of  course,  not  be  maintained,  but 
the  resistant  rocks  will  begin  to  rise  above  its  flat  surface.  This  i; 
the  origin  of  nearly  all  of  the  prominent  reliefs  rising  from  the 
plains,  such  as  ridges,  buttes,  mesas  and  mountains,  which  will  be 
described  directly.  The  sloping  plains  are  developed  only  where 
the  weathering  equals  the  work  of  the  transportation,  which  latter 
depends  on  several  factors.  In  the  main  these  determining  factors 
are  the  size  of  the  transporting  current,  and  the  mechanical  com- 
position (the  fineness  or  the  coarseness)  of  the  materials  trans- 
ported. 

(2)  Controlling  Factors. 

The  current  which  here  consists  of  what  is  known  as  sheet  flood, 
does  not  vary  a great  deal.  It  is  true  that  the  rainfall  is  slightly 
greater  in  the  mountains  and  on  their  outer  slopes  than  farther 

* Reference  as  above. 

**Keyes,  Bolson  Plains  and  the  Conditions  of  their  Existence,  Am. 
Geol.,  Vol.  34,  pp.  160-164,  September,  1904. 


12 


Bulletin  of  The  University  of  Texas. 


out,  and  this  no  doubt  makes  the  sheet  flood  slightly  greater  and 
hence  more  efficient  near  the  mountains,  and  thus  tends  to  render 
some  of  the  talus  slopes  less  steep  than  they  otherwise  would  be. 
But  the  difference  is  probably  so  slight  as  to  be  quite  negligible. 
There  is  another  circumstance  which  sometimes  renders  transpor- 
tation inefficient  by  reducing  the  sheet  flood.  If  the  ground  of 
the  plain  consists  of  such  coarse  material  as  to  allow  the  rainfall 
to  immediately  go  down  into  the  ground,  there  will  be  no  'sheet 
flood  to  perform  any  work  at  all,  and  transportation  will  be  at  a 
standstill,  except  on  comparatively  steep  slopes  where  the  work  is 
considerably  aided  by  gravitation.  A belt  of  dissected  hills,  which 
extend  northwest  from  the  Dugout  draw,  appears  to  have  been  left 
in  this  wa}q  rising  above  the  graded  plain.  They  are  locally  known 
as  the  “White  Hills”  and  consist  mostly  of  gravel  and  sand. 

The  variation  in  the  mechanical  composition  of  the  eroded  rock 
underlying  the  graded  plains  elsewhere  is  not  very  great.  In  fact, 
these  rocks  are  uniformly  so  fine  in  texture  as  to  make  their  prod- 
ucts of  disintegration  easily  transported  even  by  a slow  current. 
The  slopes  which  are  steep  enough  to  produce  such  a current  in 
the  sheet  floods  resulting  from  the  present  rainfall,  is  about  two 
feet  in  a hundred.  But  there  remains  on  the  surface  very  gen- 
erally a residue  of  coarse  material,  in  part  derived  from  concre- 
tions in  the  bed  rock  and  in  part  coming  down  from  indurated 
rocks  in  the  bordering  mountains.  This  constitutes  a land  drift 
in  a limited  sense.  Its  coarseness  has  a noticeable  relation  to  the 
slope.  Except  in  places  where  the  wind  is  at  work,  the  most  level 
parts  of  the  graded  plain  consists  of  fine  'sand  and  silt  and  no  peb- 
bles appear  on  the  surface.  On  the  moderate  slopes  and  away 
from  the  mountains  the  largest  fragments  seen  seldom  measure 
more  than  two  inches  in  diameter,  while  on  the  higher  slopes  we 
find  large  boulders. 

The  surface  of  the  graded  plains  is  more  or  less  trenched  by 
arroyos  that  traverse  them  in  the  direction  of  their  general  slope. 
We  find  these  radiating  in  all  directions  from  the  Chisos  moun- 
tains. That  the  conditions  under  which  these  plains  are  formed 
are  quite  different  from  those  attendant  on  base  leveling  is  evident 
from  these  arroyos.  They  often  run  parallel  and  close  together  for 
miles.  On  the  •southeast  side  of  the  Chisos,  Glen  draw  runs  side  by 
side  with  another  arroyo  for  twelve  miles,  and  the  two  are  mostly 


The  Geology  of  the  Chisos  Country. 


13 


less  than  a mile  apart.  Such  conditions  can  not  exist  except  where 
the  attitude  of  the  land  surface  is  well  adjusted  to  a steady  rate 
of  erosion,  and  where  it  is  yet  so  far  above  the  base  level  that  very 
little  side  cutting  is  being  done.  Under  such  conditions  the  dif- 
ferent drainage  lines  make  no  inroads  on  each  other.  The  depth 
of  the  arroyos  vary  from  almost  nothing  to  two  and  three  hundred 
feet.  We  usually  find  them  from  one-fourth  to  one-half  a mile 
apart  and  this  distance,  too,  is  proportionate  to  the  slope,  but  in- 
versely, being  least  where  the  slope  is  most  steep,  and  greatest 
where  the  slope  is  most  flat.  As  we  cross  over  the  flats  between 
them  we  notice  that  these  have  a very  gentle  slope  to  either  side 
from  a middle  divide.  It  is  evident  that  the  flats  are  being  cut 
down  by  sheet  flood  erosion,  however  slow  the  process  may  be.  In 
their  lower  course  these  long  arroyos  often  have  an  alluvial  vallej 
two  or  three  hundred  yards  wide  with  a meandering  channel,  but 
in  their  upper  course  they  are  Y-shaped. 

(3)  Maintenance  of  the  Eroded  Plain. 

With  erosion  in  general  progress,  it  may  he  asked  why  the  graded 
plains  are  not  wholly  dissected  bv  the  arroyos.  The  present  writer 
believes  that  the  answer  to  the  questions  is  to  be  found  in  the  fact 
that  weathering  in  this  region  is  very  rapid  and  transportation  com- 
paratively slow.  There  is  always  ready  and  at  hand,  wherever  the 
bedrock  consists  of  soft  cretaceous  marls  and  clays,  a surplus  of 
load  for  the  sheet  flood.  If  now  and  then  this  should  carve  a 
trench  on  the  surface  of  the  graded  plain,  this  trench  will  promptly 
be  filled  with  surplus  drift,  up  to  the  general  grade.  Thus  there 
can  be  no  trenching  done  on  the  plain  itself.  For  a similar  rea- 
son the  backing  of  the  arroyos  and  their  branches  is  exceedingly 
slow.  The  long  arroyos  are  overloaded  with  debris  from  the  moun- 
tains and  this  is  frequently  supplied  in  such  quantity  as  to  bring 
about  a re-filling  of  some  of  their  valleys.  About  midway  between 
Indianola  peak  and  the  old  Reed  camp  the  arroyos  are  now  filling 
to  such  an  extent  as  to  make  the  beds  appear  more  like  alluvial  fans 
than  like  alluvial  valleys.  Each  arroyo  maintains,  as  it  were,  a 
grade  of  its  own,  depending  upon,  the  nature  and  the  quantitv  of 
its  load,  and  this  is  usually  so  great  as  to  leave  but  little  energy  for 
other  work,  especially  in  the  upper  part  of  its  course.  It  is  onlv 
farther  down  that  wide  cutting  takes  place.  The  automatic  and 


14 


Bulletin  of  The  University  of  Texas. 


uniform  grading  of  the  plains  under  a ready  load  for  the  sheet- 
flood  current  results  in  the  prevention  of  the  water  being  gathered 
into  currents  which  might  start  re-entrant  gulleys  on  its  scarps. 
Hence  there  is  a lack  of  ramifications  in  the  drainage  valleys,  and 
hence  it  is  also  that  reduction  of  old  levels  often  proceed  by  low 
and  long  scarps  which  mark  the  boundaries  of  new  and  lower 
grades,  wherever  these  find  an  opportunity  to  establish  themselves. 

To  sum  up,  we  may  say : the  plains  around  the  Chisos  mountains 
probably  mark  the  location  of  an  old  and  higher  plain.  They  have 
been  cut  out  of  this  old  plain  by  a slow  process  of  erosion  under 
conditions  of  an  overloaded  drainage  and  have  long  since  reached 
an  equilibrium  of  grade  and  load  to  the  working  current.  The  gen- 
eral direction  of  the  grade  is  toward  the  Rio  Grande  and  this 
stream  must  have  been  the  prime  agent  in  the  destruction  of  the 
old  plain. 

3.  The  High  Reliefs. 

In  a manner,  all  of  the  high  reliefs  in  this  region  may  be  re- 
garded as  due  to  the  greater  resistance  to  erosion  by  the  more  in- 
durated rocks  in  the  faulted  and  folded  complex  of  formations 
which  have  for  a long  time  been  subjected  to  erosion.  With  the 
possible  exception  of  the  Chisos  mountains  themselves,  no  part  of 
the  topography  is  inherited  from  the  original  configuration  of  the 
land,  although  this  happens  to  be  reproduced  at  the  present  time  in 
several  places.  For  we  find  that  a fold  or  a fault  often  brings  up 
the  limestones  of  the  Lower  Cretaceous  age  above  the  surface  of 
the  graded  plains,  and  they  rise  bared  or  only  partially  destroyed 
in  their  true  structural  relations  as  ridges  or  mesas.  Thus  we  have 
the  ridges  of  the  Carmen  and  the  Santiago  ranges  and  the  Mariscal 
mountain  ridge  extending  northwestward  across  the  Rio  Grande 
toward  the  Chisos  mountains  from  the  southeast.  These  ridges 
mark  lines  of  original  uplifts.  West  of  Terlingua  creek  the  same 
rocks  form  an  extensive  tilted  mesa  and  these  again  rise  into  a 
dome  in  the  Christmas  mountains  to  the  northwest.  The  Boquillas 
flags  are  less  resistant  but  their  disintegration  never  keeps  pace 
with  that  of  the  shales  and  marls,  and  thus  we  find  them  forming 
sloping  benches  bordering  the  ridges  formed  by  the  .other  lime- 
stones on  which  they  rest.  The  sandstones  in  the  Terlingua  beds 
often  form  long,  low,  and  narrow  ridges,  where  their  tilted  edges 


The  Geology  of  the  Chisos  Country.  1 5 

are  exposed,  but  on  account  of  their  limited  thickness  they  cause 
no  high  reliefs.  The  igneous  rocks,  which  are  usually  most  re- 
sistant, form  most  of  the  high  reliefs.  Dikes  and  surface  flows,  as 
well  as  intrusives,  have  weathered  out  from  their  matrix  of  sedi- 
ments and  form  reliefs  of  most  varied  forms.  Ridges  protected  on 
one  side  by  an  intrusive  sheet  and  presenting  an  escarpment  of 
erosion  on  the  other  are  very  common.  Domes  formed  by  uplifted 
sills  and  loecolites  are  next  in  frequency.  We  also  sometimes  find 
walls  and  ridges  of  dikes  and  some  mesas  protected  by  cappings  of 
surface  flows.  Many  of  the  igneous  intrusive  bodies  are  so  slightly 
affected  by  weathering  that  they  exhibit,  almost  unchanged,  their 
original  molded  form. 

The  main  west  body  of  the  Chisos  mountains,  the  so-called  “rim 
rock,”  consists  largely  of  an  intrusive  body  which  has  once  been 
covered  by  sediments,  and  which,  so  far  as  the  present  topography 
is  concerned,  must  have  had  the  same  history  as  the  other  intru- 
bives : it  has  been  laid  bare  by  the  removal  of  superincumbent  sedi- 
ments and  volcanic  flows.  Some  of  the  latter  are  still  left  in 
patches  on  the  highest  summits,  such  as  Rummel  peak,  Lost  Mine 
peak,  Crow  mountain  and  a plateau  south  of  the  Emory  peak.  The 
fact  that  these  remain  as  a cluster  around  the  greatest  laccolite  in 
the  region  suggest  that  the  volcanic  covering  was  originally  heavier 
at  this  point  than  farther  away. 

B.  HISTORY  OF  THE  DRAINAGE. 

The  Rio  Grande  is  the  master  stream,  and  it  has  maintained  its 
course  across  a number  of  faults  and  folds.  With  regard  to  the 
geological  structure,  its  course  is  clearly  antecedent  and  was  deter- 
mined before  the  present  structures  were  all  developed.  Its  main 
tributaries  from  the  north  in  this  region  are  Terlingua  creek,  Tor- 
nillo  creek  and  Miravillas  creek.  The  first  one  drains  a large  ter- 
ritory to  the  west  and  northwest  of  the  Chisos,  and  it  ys  clearly 
subsequent  in  its  character.  It  has  adjusted  itself  to  the  larger 
geological  structures  encountered.  Thus  its  lower  course  is  evi- 
dently determined  by  the  great  Terlingua.  fault.  Tornillo  creek 
drains  the  north  and  east  slope  of  the  Chisos  and  this  stream  has 
likewise  been  influenced  by  the  geological  structure,  for  it  follows 
in  the  main  the  line  of  the  outcrop  of  the  softer  strata.  At  its  mouth 


16 


Bulletin  of  The  University  of  Texas'. 


it  has  carved  a short  canon  through  an  uplifted  limestone  and,  so 
far,  it  here  retains  an  antecedent  character.  The  Miravillas  is 
likewise  in  part  antecedent  and  in  part  subsequent.  The  main 
stream  follows  the  east  side  of  the  Santiago  and  the  Carmen 
ranges  and  has  no  doubt  been  held  in  place  by  the  great  fold  of  the 
ranges.  But  two  of  its  west  tributaries  cross  over  from  the  west 
side  of  the  fold.  These  are  antecedent.  Persimmon  gap  on  the 
Santiago  range  marks  the  former  crossing  of  another  tributary 
from  the  same  direction,  whose  upper  portion  has  later  been  cap- 
tured by  Dog  creek.  It  may  be  that  the  upper  part  of  the  Tornillo 
creek  once  drained  in  the  same  direction,  but  this  can  only  be  a 
conjecture. 

In  general  the  drainage  of  the  Chisos  country  is  in  a stage  of 
adjustment  to  geological  structure.  Nearly  all  of  the  smaller 
streams  are  through  this,  stage,  the  larger  creeks  are  still  in  it,  and 
the  Rio  Grande  has  hardly  yet  entered  upon  it. 

C.  STRATIGRAPHY. 

In  order  to  understand  the  geological  structure  of  this  country 
it  is  essential  that  we  should  first  know  the  nature  and  the  thick- 
ness of  the  strata  from  which  the  entire  land  has  been  built.  It  is 
well  known  that  the  bedded  rocks,  such  as  limestone,  clays,  marls, 
and  sandstones  have  been  laid  down  as  sediments  on  the  bottoms  of 
seas  and  lakes,  and  lay  originally  in  a horizontal  position.  The 
first  task  of  a geologist  working  in  any  region  is  to  make  out  the 
general  section,  or  succession  from  below  upward,  of  the  several 
formations  and  strata  which  he  finds  exposed  on  the  surface.  He 
must  measure  their  thickness  and  see  if  this  is  the  same  everywhere, 
or  if  it  differs  at  different  points.  He  must  make  out  if  all  of  the 
strata  are  present  as  originally  laid  down,  or  if  any  of  them  have 
been  removed  previous  to  the  deposition  of  later  beds.  When  this 
is  done,  he  will  be  able  to  understand  the  structural  features  of  the 
land.  He  will  see  where  it  has  been  elevated  or  where  it  has  been 
sunken,  where  the  strata  have  been  folded  or  where  they  have  been 
broken,  and  which  strata  have  been  cut  away  by  erosion  or  which 
lie  buried  below  the  surface. 


The  Geology  of  the  Chisos  Country. 
1.  The  General  Section. 


17 


The  oldest  sedimentary  rocks  in  this  region  belong  to  the  Palaeo- 
zoic era.  Above  these  oldest  sediments  we  find  the  Lower  Creta- 
ceous limestones,  which  in  turn  are  covered  by  various  sediments  of 
the  Upper  Cretaceous.  These  are  overlain,  in  a few  places,  by 
flows  of  lava  and  by  tuffs  of  a later  age  and  by  clays  and  gravels  of 
the  late  Tertiary.  The  most  recent  formation  consists  of  the  land 
drift  and  the  alluvium  of  the  present  stream  valleys.  These  overlie 
all  of  the  other  rocks  more  or  less  persistently. 

Within  a radius  of  twenty-five  miles  from  Emory  peak  in  the 
Chisos  mountains  no  older  rocks  than  the  Mesozoic  appear  at  the 
surface  on  the  north  side  of  the  Itio  Grande,  but  farther  north 
much  older  sediments  cover  an  area  of  at  least  700  square  miles. 
As  these  throw  important  light  on  the  structure  of  the  region,  be- 
ing the  nearest  exposures  of  rocks  which  must  underlie  the  entire 
Chisos  country,  a brief  account  of  them  is  included  in  the  general 
section,  even  though  this  account  is  necessarily  incomplete  and  the 
outcrops  lie  distant  from  the  Chisos  mountains.  The  divisions 
of  the  entire  section,  thus  more  or  less  fully  described,  is  as  given 
in  the  table  below. 


Table  Exhibiting  the  Formations  Identified  in  the  Chisos  Country. 


Group. 

System. 

Series. 

Formations. 

Caenozoic. 

Pleistocene. 

Land  drift  and  alluvium. 

Tertiary. 

Miocene  and 
Pleiocene. 

Dug-out  clays  and  gravels. 
Igneous  rocks  and  bedded 
tuff  and  gravels. 

Mesozoic. 

Cretaceous. 

U pper  Cretaceous. 

Chisos  beds? 

Tornillo  beds. 

Rattlesnake  beds. 

Terlingua  beds. 

Boquillas  flags. 

Lower  Cretaceous. 

Buda  limestone  and  Del  Rio 
clay. 

Undetermined  limestones, 
shales  and  conglomerates. 

Palaeozoic. 

Carboniferous. 

Upper 

Carboniferous. 

Cibolo  limestone. 

Ordovician. 

Middle 

Ordovician. 

Limestones,  with  cherts 
shales  and  conglomerates. 

18  Bulletin  of  The  University  of  Texas. 

(1)  The  Palaeozoic. 

In  the  immediate  vicinity  of  the  Chisos  mountains  the  Palaeozoic 
sediments  are,  as  already  stated,  buried  under  the  Cretaceous  rocks,, 
but  the  Ord  and  the  Santiago  ranges  mark  the  western  limit  of  an 
uplifted  area,  where  the  Mesozoic  rocks  have  been  removed,  and 
where  the  older  sediments  now  are  exposed.  The  nearest  outcrop 
is  in  Persimmon  gap,  where  dark  shales  and  sandstones  underlie 
the  lower  Cretaceous  limestones  unconformably.  The  area  is  ap- 
parently small  and  confined  to  the  east  end  of  the  gap.  No  fossils 
were  noted  here  and  the  age  of  these  beds  are  unknown,  except  so 
far  as  it  can  be  made  out  from  their  position. 

(2)  The  Ordovician. 

About  fourteen  miles  to  the  north  northwest  of  Persimmon  gap, 
and  nearly  due  east  of  Santiago  peak,  similar  shales  and  sandstones 
appear  again,  and  from  this  point,  as  we  travel  north,  our  road  is 
on  Palaeozoic  rocks  all  the  way  to  Marathon,  and  from  there  to 
Altuda.  Marathon  lies  in  the  center  of  a triangular  area,  where 
the  rocks  of  the  Palaeozoic  era  are  now  uncovered.  This  area  is 
bounded  on  the  west  by  the  Ord  range,  on  the  north  by  the  Glass 
mountains,  and  on  the  east  and  south  by  a less  well  defined  line, 
extending  from  the  northeast  to  the  southwest,  and  passing  some 
distance  east  of  Haymond. 

There  must  be  several  thousand  feet  of  sediments  exposed  in  this 
area.  They  have  been  folded  and  faulted  and  dip,  usually  at  high 
angles,  either  to  the  southeast  or  to  the  northwest.  My  trip  over 
this  country  ivas  hurried  and  no  attempt  could  be  made  to  measure 
the  thickness  of  these  deposits.  In  the  south  part  of  the  area  they 
consist  largely  of  dark  shales  and  limestones.  These  contain  heavy 
ledges  of  chertv  quartz,  which  rise  in  long  ridges  running  from  the 
northeast  to  the  southwest.  The  chert  is  usually  interbedded  with 
ledges  of  dark  limestone,  which  now  and  then  contain  round  and 
loaf -like  concretions  of  chert.  The  prevailing  color  of  the  chert  is 
white,  but  it  is  sometimes  black,  green,  bluish,  or  of  a brown  or  a 
pinkish  color.  With  some  of  the  limestones  there  are  thin  seams 
of  sandstones  and  conglomerates.  The  latter  usually  consist  of 
well-worn  pebbles,  mostlv  of  limestone,  but  also  of  hard  quartzite. 
In  some  shales,  which  are  associated  with  chert  and  limestones  in 


The  Geology  of  the  Chisos  Country.  19 

a ridge  on  the  south  side  of  Edward’s  creek,  about  ten  miles  north- 
east of  Santiago  peak,  there  is  one  stratum  which  contains  large 
concretionary  aggregates  of  crystals  of  barite.  Some  of  these  ag- 
gregates are  a foot  in  diameter,  and  consist  of  columnar  crystals 
radiating  from  the  centre  of  the  concretions.  The  mineral  was 
seen  to  follow  a stratum  about  three  feet  in  thickness,  and  the  con- 
cretions in  one  place  appeared  to  be  present  in  large  quantity.  The 
limestones  themselves  are  frequently  bituminous.  Some  ledges 
consist  of  small  worn  organic  fragments.  From  one  of  these  some 
fossil  fragments  were  taken  which  Dr.  Charles  Schuchert  has  iden- 
tified as  the  outer  portions  of  the  glabella  of  a Trinucleus.  Asso- 
ciated with  this  there  were  also  a Plectambonites  [ sericeusf ],  a 
Rafinesquina  and  possibly  a Zygospira,  A gasteropod  of  the  genus 
Cyclora  was  found  in  another  ledge,  and  in  still  another  limestone 
a Nodosaria  was  noted.  All  of  these  fossils  were  collected  along 
the  wagon  road  near  Ridge  spring  and  at  different  points  south 
from  this  place  for  a distance  of  ten  miles.  The  same  kind  of 
rocks  continue  north  from  this  spring  as  far  as  to  Pena  Colorado, 
four  miles  south  of  Marathon.  North  from  Pena  Colorado  we 
find  shales,  limestones,  and  thin  conglomerates,  but  no  chert  beds. 
An  Athyris,  a Fistulipora  (?),  a Dentalium,  and  joints  of  crinoid 
stems  were  observed  in  the  vicinity  of  Marathon,  where  the  dip  of 
these  rocks  is  quite  generally  to  the  northwest.  Evidently  these 
sediments  are  younger  than  the  formation  which  contain  the  chert 
south  of  Pena  Colorado. 

The  thickness  of  the  ancient  sediments,  which  are  seen  on  these 
plains  around  Marathon,  is  no  doubt  several  thousand  feet,  and 
they  very  likely  contain  formations  of  more  than  one  age.  For 
twenty-five  miles  the  road  from  the  Chisos  runs  over  the  edges  of 
beds  tilted  at  high  angles.  The  fossils  associated  with  the  chert 
show  that  some  of  these  strata  belong  to  the  Ordovician,  and  Dr. 
Schuchert  infers  that  these  are  of  the  Trenton  period.  In  his 
Physical  Geography  of  the  Texas  region  Prof.  R.  T.  Hill  expresses 
it  as  his  opinion  that  the  limestones,  shales,  etc.,  of  these  plains 
may  be  of  Lower  Helderberg  age,*  and  there  is  no  good  reason  to 
doubt  that  the  Silurian  rocks  are  represented  in  this  extensive 
complex  of  folded  strata.  Proceeding  to  the  northwest  from  Mara- 

*Physical  Geography  of  the  Texas  Region,  R.  T.  Hill,  U.  S.  Geol.  Surv. 
Topographical  Atlas  of  the  United  States,  Folio  No.  3,  p.  4. 


20 


Bulletin  of  The  University  of  Texas. 


thon  we  find  that  the  dips,  which  are  prevailingly  to  the  north- 
west, grow  less  steep.  We  are  evidently  going  away  from  an  axis 
of  uplift  and  we  are  presumably  coming  to  sediments  which  are 
later  than  those  encountered  nearer  to  this  axis. 


(3)  The  Upper  Carboniferous. 

Near  Altuda,  which  is  a station  on  the  Southern  Pacific  Rail- 
road at  the  west  corner  of  the  Marathon  Palaeozoic  area,  we  find 
shales  and  limestones  that  clearly  belong  to  the  Upper  Carbonifer- 
ous series.  In  the  Altuda  mountain  these  rocks  form  the  greater 
part  of  the  mass  and  to  the  'south  of  it  the  series  outcrop  for  a 
distance  of  two  or  three  miles.  Along  a north  and  south  line 
across  this  mountain  the  dip  is  to  the  north  and  increases  in  steep- 
ness as  we  approach  a granite  area  lying  two  miles  south  of  the 
mountain.  The  section  of  the  Carboniferous  as  seen  along  this 
line  is  about  as  follows: 

Thickness  in  feet. 

A cream-colored  limestone  almost  without  bedding  planes  and 
somewhat  crystalline  in  texture,  forming  a high  vertical 
escarpment  all  around  the  mountain.  Obscure  trace's  of 
organic  fragments  were  frequently  observed,  but  no  identi- 
fiable fossils 600 

Thin  bedded  limestone  containing  shaly  and  sandy  strata. 

The  limestone  is  mostly  in  layers  from  three  to  six  inches 
thick.  This  member  contains  Fusulina  in  great  abundance 
at  times,  but  mostly  only  very  minute  young  individuals. 
Other  fossils  noted  on  the  north  side  of  the  mountain  were : 
Chonetes,  Pugnax,  Productus,  Seininula , Meekella,  a 
pygidium  of  a trilobite,  Fistulipora,  a spine  of  a fish,  and 

joints  of  crinoid  stems 200 

A series  of  shales,  sandstones  and  limestones,  with  a con- 
glomerate near  middle,  probably  as  much  as 2000 

Fusulina  cylindrica  of  a moderately  large  size  occurs  in  pro- 
fusion in  some  limestone  ledges  in  the  upper  half  of  the  lowest 
division.  Near  its  middle  there  is  a Conglomerate  twenty  feet  in 
thickness,  which  contains  pebbles  as  much  as  three  inches  in  di- 
ameter. The  largest  consist  for  the  most  part  of  a fine  brown 
quartzite,  while  those  of  smaller  size  are  of  white  quartz  and 


The  Geology  of  the  Chisos  Country. 


21 


black  chert.  All  are  well  rounded.  The  conglomerate  is, 
cemented  by  a dark  gray  calcareous  stony  material,  which  was  seen 
to  contain  some  shells  of  brachiopods,  some  bryozoa,  and  a coral. 
Some  hundred  feet  or  more  under  this  conglomerate  there  is  a 
dark  limestone  which  occasionally  contains  numerous  specimens  of 
an  ammonoid,  and  associated  with  this  were  noted  some  stems  of 
crinoids,  some  gasteropods,  and  an  Archeocidaris. 

This  section  is  the  equivalent  of  the  Cibolo  limestone  at  Shatter 
in  Presidio  county.  It  probably  extends  down  into  the  Alta  beds, 
which  underlie  the  Cibolo  limestone.  The  thin-bedded  limestone 
described  in  the  section  above  appears  in  a ravine  to  the  north- 
west of  Altuda  mountains  and  the  face  of  this  outcrop  is  almost 
in  itself  a proof  of  the  identity  of  this  limestone  with  the  thin- 
bedded  limestone  of  the  Cibolo  formation  near  Shatter.  Another 
evidence  of  the  identity  of  the  two  is  the  presence  at  both  places 
of  a peculiar  bead-like  fossil  stem  of  unknown  relationship. 

Whether  beds  of  the  Lower  Carboniferous  and  of  the  Devonian 
ages  will  be  found  between  Altuda  mountain  and  Pena  Colorado, 
or  whether  there  is  an  unconformity  between  the  Upper  Carbonif- 
erous and  the  older  formations  is  not  yet  known. 

(4)  Rocks  of  the  Cretaceous  Age. 

Some  time  after  the  mountain  folds  of  these  older  sediments 
had  been  leveled  down  by  extensive  erosion  the  region  was  again 
submerged  and  a new  succession  of  sediments  were  laid  down  in  an 
open  sea,  which  is  known  to  have  had  a continuous  extension  from 
Canada  to  Mexico  and  which  covered  Western  Texas  during  nearly 
the  entire  Cretaceous  age.  The  sediments  in  this  sea  have  a great 
thickness,  no  less  than  8000  feet  in  Western  Texas.  They  consist 
of  heavy  limestones  with  some  shale  and  conglomerate  below; 
flaggy  limestones,  chalky  rock,  marls,  and  clays  in  their  middle 
portion;  and  sandstones,  clays  and  tuffaceous  beds  above.  We  will 
first  describe  the  lower  division. 

The  Loiuer  Cretaceous  Limestones. 

In  the  Carmen  and  Santiago  ranges,  in  the  Christmas  and  the 
Mari  seal  mountains,  and  in  the  Mesa  de  Anguila  the  Lower  Cre- 
taceous limestones  are  extensively  exposed,  but  the  whole  succes- 


22 


Bulletin  of  The  University  of  Texas. 


sion  of  all  the  beds  has  not  been  seen.  It  is  estimated  that  a 
thickness  of  more  than  2000  feet  appears  in  the  Carmen  range. 
At  the  mouth  of  the  Santa  Helena  canon  the  Lower  Cretaceous 
limestones  rise  1500  feet.  But  at  neither  of  these  places  is  the  base 
of  the  formation  exposed.  This  comes  to  view  in  Persimmon  Gap 
and  elsewhere  farther  north  in  the  Ord  and  the  Santiago  ranges 
and  on  the  borders  of  the  Marathon  Palaeozoic  area,  where  only 
incomplete  measurements  have  been  made. 

Persimmon  Gap. 

At  Persimmon  gap  the  lowest  100  feet  of  the  Lower  Cretaceous 
consist  of  ’shaly  calcareous  strata,  which  are  not  well  exposed. 
Above  this  lies  a sharply  marked  conglomerate,  about  twenty  feet 
in  thickness,  consisting  of  well-worn  pebbles,  mostly  of  chert.  The 
ledge  may  be  seen  for  several  miles  to  the  north  in  the  east  face  of 
the  range. . Above  it  there  are  two  or  three  hundred  feet  of  lime- 
stone, some  ledges  of  which  were  seen  to  contain  numerous  im- 
bedded tests  of  Orbitulina  texana,  Poem. 

Altuda. 

Another  place  where  the  basal  members  of  the  Lower  Cretaceous 
may  be  seen  is  in  the  high  limestone  scarp  west  of  Altuda,  about 
seventy-five  miles  north  of  the  Chisos  mountains.  At  this  place  it 
rests  on  the  undisturbed  upper  surface  of  the  Cibolo  limestone  of 
the  Chinati  'section.  It  consists  of  gray  ledges  of  limestone  only 
slightly  different  from  the  underlying  formation.  The  boundary 
between  the  two  is  marked  in  some  places  by  occasional  fillings  of 
yellow  sandstone  which  appear  at  the  base  of  the  formation  in  de- 
pressions in  the  upper  surface  of  the  Cibolo  limestone,  and  else- 
where it  can  be  recognized  only  by  a few  well-worn  pebbles  of 
quartz,  which  may  usually  be  found  at  the  contact  of  the  two 
limestones.  The  conglomerate  and  the  shale  seen  farther  south 
are  absent  at  this  place.  The  succeeding  hundred  feet  are  like 
the  basal  layers,  consisting  of  gray  limestone  in  moderately  heavy 
ledges.  These  contain  a large  Pecten  with  complex  folds,  a large 
Gaprotina,  some  gasteropods,  Modiola  pedernalis  and  a Cardium. 
Above  this  are  some  forty  feet  of  stronger  and  heavier  ledges, 
which  contain  silicified  shells  of  a Radiolithes  (like  sauvagesii, 


The  Geology  of  the  Chisos  Country. 


23 


D70rb)  in  abundance  and  also  a Caprotim a large  Pecten , prob- 
ably P.  duplicosta  Roem.,  and  various  gasteropods.  Again,  above 
this  there  are  some  sixty  feet  of  softer  and  thinner  ledges  of  lime- 
stone. These  are  overlain  by  several  hundred  feet  of  heavy-bedded 
white  limestone,  the  upper  ledges  of  which  are  identical  with  the 
Edwards  limestone  of  the  south  central  part  of  the  State.  Near 
the  base  of  this  division  some  gasteropods  and  Gryphaea  marcoui 
Hill  & Vaughn  were  noted.  The  uppermost  two  hundred  feet  in 
the  Cretaceous  section  here  appear  west  of  Ramsay  creek  and  from 
two  to  five  miles  south  of  Strobel.  They  consist  of  more  shaly 
limestones,  in  which  no  fossils  were  noted.  In  all,  this  section 
measures  at  least  800  feet.  There  is  here  an  overlap  on  a height 
•of  an  old  land,  where  the  lowermost  beds  of  the  Cretaceous  sys- 
tem, in  which  conglomerate  appears  elsewhere,  were  never  laid 
down. 

Gienega  Mountain. 

The  base  of  the  Lower  Cretaceous  also  appears  flanking  the 
dome-like  uplift  of  the  Cienaga  mountains.  The  lowermost  ledges 
here  consist  of  sandstone  and  are  overlaid  by  shales  and  shaly 
limestones,  measuring  about  150  feet.  An  impression  of  the  stem 
of  a plant  was  seen  in  the  calcareous  shales  on  the  west  side  of 
the  mountain.  Above  these  there  is  a well  developed  conglomeritic 
sandstone  about  twenty-five  feet  in  thickness.  This  contains 
streaks  of  pebbles  which  measure  two  inches  in  diameter.  Most  of 
the  larger  pebbles  consist  of  white  quartz,  but  there  is  also  some 
dark  chert.  Above  this  conglomerate  there  are  some  300  feet  of 
somewhat  impure  limestones  with  shaly  seams.  In  the  lower  part 
of  these  a Caprina,  a Nerinea,  a Radiolithes  and  a Cardium  were 
noted.  On  the  north  side  of  the  mountain  this  division  is  over- 
lain  by  several  hundred  feet  of  massive  pure  limestone  ledges,  evi- 
dently corresponding  to  the  Edwards  limestone.  The  whole  series, 
so  far  as  seen  here,  has  been  heated  by  intrusives  and  turned  into 
a fine-grained  marble. 


Mesa  de  Anguila. 

The  section  in  the  great  east  escarpment  of  the  Mesa  de  Anguila 
measures  about  1500  feet.  It  is  seen  to  consist  of  about  700  feet 
of  massive  limestone  below,  rising  in  an  almost  vertical  wall,  then 


24 


Bulletin  of  The  University  of  Texas. 


about  200  feet  of  an  impure  and  more  readily  disintegrating  rock 
which  contains  some  marly  layers,  and  weathers  into  a slope,  and 
highest  up  600  feet  more  of  massive  ledges  of  white  limestone  rise 
in  a nearly  vertical  wall  to  the  top  of  the  escarpment.  Only  the 
lowermost  100  feet  were  examined  by  the  writer  at  the  mouth  of 
the  canon.  Just  above  the  water  in  the  river  is  a dark,  almost 
black,  limestone  of  very  compact  texture.  It  is  in  straight  ledges 
and  there  are  two  or  three  layers  of  shale  interbedded.  One  of 
the  limestone  ledges  was  almost  filled  with  a small  Gryphaea, 
probably  G.  mar  coni  Hill  & Vaughn.  A large  Pecten  and  an 
Exogyra  texana  Roem.,  like  one  occurring  in  the  Presidio  beds 
near  Shatter,  were  also  found  at  this  horizon. 

Christmas  Mountain. 

In  the  Christmas  mountain  there  is  a thickness  of  about  1000 
feet  of  these  limestones.  The  same  divisions  are  indicated  here: 
two  heavy  bedded  and  strong  limestones  with  a somewhat  less  en- 
during division  between  them.  Rut  the  lowest  division  is  not 
wholly  exposed.  At  the  south  end  of  the  mountain  the  same 
Badiolithes  occurs  that  was  seen  west  of  Altuda.  Farther  to  the 
north  and  on  the  west  side  I found  a cast  of  a large  gasteropod,  a 
Globichoncha  planata  Eoemer,  a Cerithium  proctori  Cragin,  and 
also  another  Cerithium. 


Mariscal  Mountain. 

To  the  south  of  the  Chisos  the  Lower  Cretaceous  has  been  lifted 
up  and  bared  in  the  Mariscal  mountain,  which  is  cut  across  bv 
the  Rio  Grande  and  extends  eight  miles  north  of  the  canon  of  this 
river.  On  the  west  slope  of  this  ridge  just  to  the  north  of  the 
west  end  of  the  canon,  the  contact  between  the  Boquillas  flags  and 
these  limestones  is  well  exposed  and  is  very  clearly  marked.  There 
is  an  abrupt  change  from  a heavy  bedded  strong  limestone  to  the 
flaggy  layers  of  the  upper  formation.  This  is  the  dividing  plane 
between  the  upper  and  the  lower  Cretaceous.  The  upper  100  feet  of 
the  lower  series  were  hastily  examined.  The  highest  ledge  consists 
of  a rock  of  very  compact  texture  exactly  like  the  Buda  limestone 
as  it  appears  around  Terlingua.  Under  a lens  this  rock  is  seen 
to  consist  of  a matrix  of  exceedingly  small  and  indistinct  frag- 


The  Geology  of  the  Chisos  Country.  25 

ments  in  which  lie  imbedded  occasional  fragments  of  shells  of 
gasteropods  and  also  some  shells  of  a Glohigerina.  About  fifty 
feet  below  the  uppermost  ledge  the  rock  weathers  more  rapidly 
and  is  less  pure  and  compact.  This  apparently  represents  the 
zone  of  the  Del  Rio  clay.  Some  fifty  feet  still  farther  down  I 
found  a large  Nerinea,  having  a spire  two  inches  wide,  and  a cast 
of  a Lunatia  ( ?) . Not  far  from  the  same  place  Kingena . wacoensis 
Roemer,  and  Ostrea  cannata  Lam,  were  noted,  and  with  these 
was  seen  imbedded  in  one  of  the  ledges  some  peculiar  meandering 
dark  plates,  that  on  closer  examination  proved  to  be  a sponge.  On 
the  east  side  of  this  ridge  and  from  two  to  three  miles  south  of 
Lindsey’s  mine,  the  uppermost  compact  limestone  is  about  seventy 
feet  thick  and  weathers  out  quite  distinctly  by  the  removal  of  more 
readily  crumbling  rock  below  it.  This  latter  contains  layers  full 
of  fragments  of  shells  and  occasional  specimens  of  Nodosaria  t ex- 
ana  Conrad  and  of  a Gryphaea  like  G.  mucronata  Gabb.  It  will 
be  remembered  that  these  two  fossils  occur  in  the  Del  Rio  zone. 
In  the  solid  ledges  below  this  zone  Requienia  patigiata  White  is 
found  associated  with  the  same  gasteropods  that  were  seen  on  the 
opposite  side  of  the  mountain,  and  some  distance  still  further  down 
Exogyra  texana  Roem.  was  noted.  In  the  canon  of  the  San  Vin- 
cente mountains  the  same  formations  are  exposed,  hut  only  to  a 
small  extent. 

The  Carmen  Range. 

The  Carmen  range  consists  of  ridges  of  limestone  of  the  Lower 
Cretaceous.  In  high  escarpment  on  the  east  side  it  appears  that 
they  show  a thickness  of  somewhat  more  than  2000  feet.  The  Rio 
Grande  cuts  across  these  ridges  by  the  Boquillas  canon,  and  this 
canon  would  show  the  best  exposures  for  a continuous  section. 
But  in  this  survey  it  was  not  practical  to  take  that  route,  and 
only  a very  general  idea  of  the  section  can  be  given.  It  resembles 
the  section  in  the  Mesa  de  Anguila,  hut  it  is  heavier.  The  ledges 
corresponding  to  the  Buda  and  the  Del  Rio  zones  weather  out  on 
the  west  slope  and  are  only  some  sixty  or  seventy  feet  in  all.  Be- 
low this  is  a heavy  bedded  limestone  some  six  hundred  feet  thick, 
in  the  upper  part  of  which  I saw  Ostrea  munsoni  Hill.  Other- 
wise recognizable  fossils  in  this  division  are  rare.  Under  this 
there  is  a considerable  thickness  of  ledges  which  weather  somewhat 


26  Bulletin  of  The  University  of  Texas. 

more  readily  and  which  contains  Exogyra  texana  Roem.,  E.  math- 
eroniana  Conrad,  a Gryphea , Pecten  texanus  Roem.  (?)  and  Badi- 
olithes  cf.  Sauvagesii  D’Orb.  These  fossils  were  seen  along  the 
trail  through  the  Ernst  valley,  eight  or  ten  miles  north  of  the  R'j 
Grande.  In  the  right  bank  of  Heath  creek,  where  it  cuts  th  ough 
the  last  of  the  Carmen  ridges,  we  find  the  same  strata  that  were 
noted  at  the  mouth  of  the  Grand  canon : dark  limestone  with  some 
shaly  strata  and  containing  Gryphea  marcoui  Hill  & Vaughn.  At 
a somewhat  higher  level  in  the  east  face  of  the  escarpment  and 
about  three  miles  to  the  south,  a Nautilus  (near  texanus  Schum.) 
was  noted.  At  the  lower  mouth  of  the  Boquillas  canon  the  lowest 
rock  seen  consists  of  a very  compact  and  close-grained  limestone, 
in  which  tests  of  foraminifera  are  very  numerous  and  in  which  a 
Gryphea  and  a small  Pecten  were  noted.  This  rock  is  very  much 
fissured  and  shows  numerous  veins  of  a columnar  calcite,  varying 
in  width  from  two  inches  to  two  feet.  Above  this  there  is  at  least 
1500  feet  of  the  heavy  bedded  limestone  which  forms  all  of  the 
ridges  in  this  range,  and  at  this  place  the  variations  of  the  ledges 
seen  at  different  levels  elsewhere  are  not  very  apparent. 

At  the  upper  mouth  of  the  Boquillas  canon  just  below  the  old 
Mexican  village  of  the  same  name,  there  are  a thousand  feet  of 
massive  yellowish-gray  limestone  with  some  dark  and  less  heavy 
ledges  below.  A very  compact  and  close-textured  gray  ledge 
shows,  when  examined  in  a thin  section,  that  it  is  essentially  an 
indurated  ooze,  in  which  minute  particles  of  organic  calcareous 
materials  make  up  the  greater  part.  Few  of  these  particles  are 
large  enough  to  indicate  the  nature  of  the  organisms  from  which 
they  are  derived,  but  there  are  some  tests  of  foraminifera.  Such 
tests  are  rather  common  in  the  entire  series  except  in  some  ledges 
which  have  undergone  secondary  changes  and  have  become  gran- 
ular and  somewhat  crystalline  in  texture.  At  this  same  place 
they  were  noted  in  great  numbers  in  a ledge  lying  about  400  feet 
higher  up  in  the  section. 

Cliaractei'istic  Horizons. 

It  was  not  attempted  to  make  out  the  succession  of  separate 
divisions  in  the  Commanche  Series,  for  this  would  have  required 
much  more  time  than  was  at.  our  disposal,  but  it  may  neverthe- 


The  Geology  of  the  Chisos  Country. 


27 


less  be  worth  the  while  to  sum  up  the  observations  made  bearing 
on  this  subject. 

1.  The  base  of  the  series  consists  of  sandy  or  clayey  beds  con- 
taining ealcareous  material  also,  weathering  rather  easily.  At 
Cienega  mountain  these  strata  have  a thickness  of  about  150  feet. 

2.  At  about  150  feet  above  the  base  there  is  a conglomerate  or  a 
pebbly  sandstone  measuring  some  twenty  feet.  These  two  mem- 
bers, and  very  likely  a part  of  the  succeeding  beds  above,  are  ab- 
sent at  Altuda,  where  they  probably  never  were  laid  down,  owing 
to  an  overlap. 

3.  At  an  unknown  distance  above  this  conglomerate  there  are 
some  dark  ledges  of  very  compact  limestone  interbedded  with  some 
shaly  seams,  and  these  are  followed  upward  by  several  hundred 
feet  of  heavy  bedded  limestones  of  lighter  color.  These  lowest 
three  divisions  are  equivalents  of  the  Presidio  beds  in  the  Shatter 
section. 

4.  Then  follow  two  or  three  hundred  feet  of  more  thin-bedded 
and  less  pure  limestones,  which  weather  more  easily  and  contain 
a fauna  resembling  that  found  in  the  Shatter  beds. 

5.  Above  this  there  are  several  hundred  feet  of  very  massive 
light  limestone,  which  is  clearly  the  equivalent  of  the  Edwards 
limestone. 

6.  The  last  and  highest  of  the  series  is  a compact  limestone  only 
some  half  a hundred  feet  thick,  which  is  the  equivalent  of  the 
Buda.  It  is  separated  from  the  massive  beds  below  by  some  twenty 
feet  of  more  easily  weathering  ledges  which  are  frequently  filled 
with  organic  fragments  and  which  corresponds  to  the  Del  Rio  clay. 

At  Terlingua  both  the  Buda  and  the  Del  Rio  zones  are  much 
heavier  than  to  the  east  of  the  Chisos.  These  formations  clearly 
thin  out  or  entirely  disappear  in  this  direction. 

Chemical  Composition. 

The  limestones  of  the  Lower  Cretaceous  are  usually  quite  pure 
and  this  is  true  not  only  of  the  most  massive  ledges,  but  also  of 
those  of  less  development,  as  is  evident  from  the  analysis  given 
belowr. 


28 


Bulletin  of  The  University  of  Texas. 


Analysis  of  Limestone  from  the  Buda  Horizon,  One  Mile  East  of 

Boquillas. 


Silica 2.35 

Alumina 21 

Ferric  oxide 24 

Lime 53.90 

Magnesia 15 

Carbonic  acid 42.23 

Water  [hygroscopic]  18 

Water  [combined]  33 

Sulphur trace 


99.59 

Cli  aract eristic  W eath ering . 

The  surface  of  the  limestones  of  the  Lower  Cretaceous  is  some- 
times etched  in  a manner  that  seems  to  be  peculiar  to  the  climatic 
conditions  of  the  Southwest.  On  top  of  the  highest  ridges  of  this 
rock,  and  elsewhere  when  these  limestones  lie  bare,  we  often  find 
small  hollows  or  basins,  not  altogether  unlike  so  called  pot-holes. 
But  they  have  clearly  not  been  produced  by  any  process  of  wear- 
ing. They  have  been  made  by  a unique  process  of  etching,  which 
takes  place  after  showers  and  rains.  At  such  times  water  is  re- 
tained in  them  for  a short  period  and  some  of  the  lower  algae 
flourish  in  this  water,  while  it  lasts.  These  produce  an  abundance 
of  carbon  dioxide  which  dissolves  the  rock.  Between  rains  this 
vegetation  dries  up,  and  can  often  be  found  as  a dessicated  crust 
of  black  mud  on  the  bottom  of  the  basins,  ready  to  take  up  the 
work  after  the  next  shower.  The  Spanish  name  “tinahita”  seems 
an  appropriate  one  for  such  basins.  It  is  % the  diminutive  of 
Tinaha,  the  name  of  the  common  waterholes  produced  by  cor- 
rosion in  creeks  and  canons. 

The  Tinahitas  always  have  a flat  or  only  slightly  concave  bottom 
with  vertical  or  even  overhanging  sides,  sometimes  beautifully 
etched.  They  vary  from  three  inches  to  six  feet  in  diameter  and 
from  half  an  inch  to  two  feet  in  depth.  The  common  dimensions 
are  a foot  in  width  and  three  or  four  inches  in  depth. 


The  Geology  of  the  Chisos  Country.  29 

The  Upper  Cretaceous  Series. 

At  the  top  of  the  upper  ledges  of  the  Lower  Cretaceous  lime- 
stones there  is  quite  a marked  change  in  the  character  of  the  sedi- 
ments of  the  age.  The  making  of  the  heavy-bedded  limestones 
seem  to  have  come  to  a sudden  end,  and  beds  of  this  kind  were  not 
laid  down  again  during  this  age  in  West  Texas,  There  is  yet 
some  more  limestone  above,  but  this  lies  in  thin  flaggy  beds  and  it 
contains  a noticeable  admixture  of  fine  siliceous  sand  and  clay. 
Later  on  the  conditions  changed  so  that  chalk,  marl,  and  clay  were 
laid  down  and  still  later,  sand  and  more  clay,  and  at  last,  sedi- 
ments containing  a large  amount  of  tuffaceous  material.  The  de- 
posits of  each  of  these  -several  and  successive  periods  must  now  be 
described. 

The  Boquillas  Flags. 

In  the  vicinity  of  Boquillas  postoffice,  all  along  the  west  flank 
of  the  Carmen  range,  the  beds  immediately  overlying  the  Lower 
Cretaceous  limestones  are  probably  exposed  to  the  best  advantage 
for  study.  These  beds  also  lie  all  around  both  the  Mariscal  moun- 
tain, and  on  the  south,  east,  and  north  sides  of  Christmas  moun- 
tain. West  of  Terlingua  creek  they  extend  from  Cuesta  Blanca  to 
the  Keed  plateau,  and  south  of  Terlingua,  still  farther  west  be- 
yond the  limits  of  the  Chisos  region.  From  two  measurements 
made  about  three  miles  east  of  Terlingua  and  one  measurement 
taken  on  the  west  side  of  Mariscal  mountain,  the  thickness  of  these 
beds,  which  we  will  call  the  Boquillas  flags,  is  585  feet  or  a little 
less  than  600  feet.  They  seem  to  have  a quite  uniform  develop- 
ment and  these  three  measurements  did  not  vary  by  more  than  50 
feet,  which  perhaps  is  within  the  limit  of  probable  error. 


Chemical  Composition. 

The  mixed  character  of  the  sediments  forming  the  Boquillas 
flags  is  believed  to  be  fairly  constant,  and  the  analyses  given  below 
are  from  typical  specimens  of  rock,  except  that  the  ledge  from 
which  the  first  one  was  taken  contained  a more  than  normally 
high  per  cent  of  organic  matter.  In  the  specimen  from  near  the 
Colquitt-Tigne  mine  sonie  of  the  silica  has  probably  been  intro- 
duced secondarily  bv  infiltration. 


80 


Bulletin  of  The  University  of  Texas. 
Analyses  of  Limestone  from  the  Boquillas  Flags  * 


(Juesta  Blanca.  Near  Colquitt-Tigne  Mine. 

Silica 7.80  per  cent.  20.72  per  cent. 

Alumina  . . 1.30  per  cent.  0.16  per  cent. 

Ferric  oxide 1.30  per  cent.  .45  per  cent. 

Lime 49.20  per  cent.  43.15  per  cent. 

Magnesia 0.15  per  cent.  0.52  per  cent. 

Carbonic  acid  38.50  per  cent,  34.20  per  cent. 

Organic  matter 1.10  per  cent 

Water  (hygroscopic)  . . 0.20  per  cent.  0.10  per  cent. 

Water  (combined)  ....  0.50  per  cent.  0.32  per  cent. 


100.05  per  cent.  99.62  per  cent. 

Clastic  Texture. 

The  Boquillas  flags  consist  of  a copious  matrix  of  very  fine 
calcareous  particles,  sometimes  so  fine  as  to  make  the  rock  appear 
to  be  without  structure,  but  containing  some  organic  fragments  up 
to  one-half  millimeter  in  diameter.  With  this  some  fine  graihs  of 
quartz  are  usually  imbedded,  seldom  exceeding  one-fourth  of  a 
millimeter  in  diameter,  and  there  are  always  present  some  shells  of 
foraminifera.  The  admixture  of  sand  varies  considerably  in  quan- 
tity, and  there  are  some  thin  layers  which  may  consist  of  as  much 
as  one-fourth  sand.  The  shells  of  the  foraminifera  are  sometimes 
crushed  flat  and  at  other  times  entire  and  filled  with  crystalline 
calcite.  Small  fragments  of  the  shells  of  Inoceramus  are  quite 
common,  and  may  be  recognized  by  their  vertical  prismatic  struc- 
ture. 

Field  Appearances. 

In  their  field  appearance  the  thin-bedded  and  flaggy  habit  is 
the  most  constant  characteristic  of  these  beds.  The  ledges  of 
which  the  formation  is  built  up  average  from  four  to  eight  inches 
in  thickness.  These  ledges,  or  layers,  are  separated  bv  delicate 
seams  which  may  not  appear  in  freshly  exposed  faces.  Occasion- 
ally in  the  lower  part  of  the  formation  there . are  ledges  which 
measure  a foot  through.  As  we  follow  them  upward  the  layers 


O.  H.  Palm,  Analyst. 


The  Geology  of  the  Chisos  Country. 


31 


become  gradually  more  thin.  At  certain  horizons  they  have  a 
chalky  texture.  This  is  especially  true  about  the  uppermost  hun- 
dred feet.  Another  habit  which  is  quite  characteristic  is  the  man- 
ner in  which  the  flags  have  been  aifected  by  joints.  They  always 
break  into  blocks  with  straight  faces  and  sharp  angles,  and  the 
three  diameters  of  these  blocks  are  of  a somewhat  constantly  un- 
equal length.  The  blocks  are  always  considerably  longer  than 
wide  and  they  have  a greater  width  than  thickness.  These  pro- 
portions of  the  three  dimensions  are  the  same  whether  the  blocks 
are  large  or  small.  One  may  find  blocks  which  are  four  feet  long 
and  no  more  than  eight  inches  wide  and  four  or  five  inches  thick. 
The  color  of  the  beds  is  quite  variable.  A cream-grayish  white  is 
the  characteristic  shade.  In  the  Boquillas  region  they  usually  have 
a faint  ferruginous  red  stain,  often  quite  marked.  On  Cuesta 
Blanca,  west  of  Terlingua  creek,  they  are  dark  and  almost  black 
on  fresh  fractures  in  some  ledges.  The  same  color  was  noted 
north  of  Christmas  mountains  in  the  Corizones  and  on  the  north 
side  of  the  Rosillos  mountains. 

Resistance  to  Erosion. 

The  Boquillas  flags  are  less  resistant  to  weathering  and  erosion 
than  the  heavy  limestone  which  underlie  them.  This  is  no  doubt 
chiefly  due  to  their  thinner  bedding  and  more  close  jointing,  both 
of  which  properties  cause  them  to  break  into  smaller  blocks  that 
yield  more  readily  to  the  agencies  of  disintegration.  Around  the 
flanks  of  the  uplifted  domes  and  ridges  of  the  underlying  lime- 
stones the  tilted  edge  of  this  formation  usually  rises  only  a part 
of  the  way  up,  leaving  the  upper  surface  of  the  older  rock  bare. 
To  underground  waters  it  is,  however,  more  resistant  than  the 
purer  rock  on  which  it  rests.  On  account  of  the  clay  constituent 
which  it  contains,  it  is  more  impervious  to  water  and  hence  it  very 
rarely  contains  convernous  fissures,  and  for  the  same  reason,  also, 
the  contact  between  these  two  formations  near  Boquillas  marks  a 
line  of  hot  springs,  where  waters  coming  from  below  are  carried 
out  to  the  nearest  point  of  emergence  of  the  base  of  the  overlying 
less  easily  penetrated  formation. 


32  Bulletin  of  The  University  of  Texas. 

Minor  Characteristics. 

One  or  two  other  characteristic  features  should  perhaps  not  be 
passed  by.  On  the  upper  surface  of  'some  of  the  ledges  clusters  of 
cubic  crystals  of  limonite  are  to  be  seen.  These  are  pseudomorphs 
after  pyrites,  which  sometimes  have  formed  in  the  dark  ledges.  A 
two-inch  seam  of  amorphous  limonitic  material  was  noted  in  the 
upper  part  of  the  beds  south  of  Cuesta  Blanca. 

At  several  places  there  were  found  on  some  of  the  layers  in 
these  beds  a peculiar  kind  of  markings,  which  consist  of  straight 
grooves  when  on  the  upper  surface  of  a layer,  or  straight  raised 
lines  when  on  the  lower  surface.  They  measure  from  one-half  to 
two  or  three  inches  in  length  and  are  usually  connected  by  similar 
but  diverging  structures  at  one  or  more  points.  Sometimes  they 
are  crowded  together  into  a network.  These  structures  have  a 
marked  resemblance  to  some  markings  observed  on  a ledge  of  sand- 
stone in  the  Middle  Cretaceous  on  the  south  side  of  the  Black 
Hills  in  South  Dakota  and  described  as  fossil  frost  cracks.*  What- 
ever their  origin  may  be,  they  are  quite  peculiar  in  appearance 
and  certainly  different  from  the  structure  generally  known  as  mud 
cracks.  Their  occurrence  at  the  same  horizon  in  two  as  widely 
separated  localities  as  the  Black  Hills  and  the  Chisos  mountains 
entitles  them  to  our  notice.  The  present  writer  has  also  noted 
their  occurrence  in  the  Boquillas  flags  at  Ochinaga,  in  Mexico,  and 
in  Val  Verde  and  Kinney  counties  in  Texas. 


Fossils. 


Though  rich  in  individuals  of  a few  species,  the  fauna  of  these 
beds  is  meager  in  the  number  of  species.  No  close  search  was 
made  for  fossils  and  this  circumstance  no  doubt  helps  to  make  the 
list  of  forms  which  were  observed  very  small.  It  is  as  below : 

1.  Globigerina. — -Several  species  were  noted  in  one  of  four 
rock  specimens  which  were  examined,  and  one  or  two  species  were 
seen  in  the  other  three.  This  shell  occurs  everywhere  throughout 
the  whole  formation. 

2.  Textularia. — This  is  less  abundant  than  Globigerina  but  is 
usually  found  with  it. 

*See  Scientific  American,  Vol.  LXXII,  p.  102.  1805. 


The  Geology  of  the  Chisos  Country. 


33 


3.  Small  ammonoid,  undetermined. 

4.  Inoceramus  confertim-annulatus  Roem.: — This  is  the  most 
common  fossil  in  the  formation  and  occurs  throughout  its  extent, 
apparently  increasing  'somewhat  in  size  from  below  upward. 

5.  Inoceramus  exogyroides  Meek.  Rare. 

6.  Inoceramus  umbonatus  Meek.  Infrequent. 

7.  Crioceras  cf.  latus  Gabb. — This  fossil  is  quite  common  on 
the  upper  surface  of  one  or  two  somewhat  indurated  ledges  a little 
above  the  middle  of  the  formation. 

8.  A cycloid  fish  scale,  two  inches  in  diameter,  was  noted  on  the 
upper  surface  of  a rather  thick  ledge  of  limestone  in  the  bottom 
of  an  arroyo  near  the  Colquitt-Tigne  mine. 

Correlation. 

The  Boquillas  flags  are  the  western  equivalent  of  the  Eagle 
Ford  shales  in  the  east  part  of  the  State.  Though  they  are  more 
calcareous  in  this  region  than  farther  east,  the  resemblance  of  the 
two  is  quite  evident.  Both  are  made  up  of  thin  and  straight  layers, 
separated  by  thin  clayey  seams.  Both  contain  sand  and  bitumin- 
ous material.  And  both  follow  directly  on  top  of  the  Buda  lime- 
stone. There  is,  however,  a difference  in  the  fauna.  In  the  east 
part  of  the  State  sharks5  teeth  and  large  ammonoids  characterize 
the  Eagle  Ford  horizon,  but  these  seem  to  be  absent  here.  The 
physical  conditions  were  evidently  not  quite  the  same.  In  East 
Texas  the  formation  has  a much  smaller  development.  The  shore 
of  the  middle  cretaceous  sea  must  have  been  nearer  there  and 
deposition  lasted  for  a shorter  period.  Indeed,  there  is  some  evi- 
dence of  work  of  submarine  currents  strong  enough  to  have  pre- 
vented sedimentation  for  some  time  after  the  Buda  stage.  Nb 
such  conditions  were  noted  in  the  Chisos  country  and  thus,  prob- 
ably, this  formation  here  reached  a greater  development  and  came 
to  have  a somewhat  different  aspect,  faunally  as  well  as  physically. 

The  Terlingua  Beds. 

There  was  no  very  abrupt  change  attending  the  physical  con- 
ditions which  brought  the  Boquillas  epoch  to  a close.  The  upper 
hundred  and  fifty  feet  in  the  Boquillas  beds  contain  a very  consider- 
able amount  of  chalky  ledges,  only  slightly  more  indurated  than 


34 


Bulletin  of  The  University  of  Texas. 


the  deposits  of  the  following  epoch,  which  first  consist  of  a yellow- 
ish white,  indurated,  stratified  chalk.  This  gradually  changes  up- 
ward to  an  impure  gray  marl,  which  becomes  less  and  less  calca- 
reous, until  it  is  a true  clay.  In  the  uppermost  strata  of  these 
clays  there  are  some  thin  layers  of  concretionary  limestone  and 
calcareous  sandstones.  We  shall  designate  this  entire  'succession 
of  somewhat  heterogeneous  deposits  “the  Terlingua  beds,”  as  they 
are  well  exposed  along  the  creek  which  bears  this  name.  From  a 
structural  as  well  as  from  a topographic  point  of  view  the  differ- 
ent members  of  which  it  is  made  up  are  practically  a unit.  In 
all  they  comprise  a thickness  of  about  1250  feet. 


Distribution. 

As  the  Terlingua  beds  rest  on  the  Boquillas  flags,  we  always 
find  them  outside  of  the  latter  in  going  from  centers  of  local  up- 
lifts. The  nearest  lowlands  around  the  mountains  and  hills  of  the 
Lower  Cretaceous  limestones  are  mostly  overlaid  by  readily  yield- 
ing clays  of  this  formation.  This  habit  is  probably  best  illustrated 
in  the  case  of  Mariscal  mountain,  southeast  of  the  Chisos,  where 
these  beds  underlie  a belt  from  one-half  to  two  miles  wide,  on  all 
sides  of  the  ridge.  A 'similar  belt  follows  around  the  north  end 
of  San  Vincente  mountain  and  then  turns  around  and  follows  the 
west  side  of  the  Boquillas  range  northward.  There  is  likewise  an 
irregular  belt  coming  around  the  Christmas  mountain  on  the  east. 
Along  the  Terlingua  creek  they  underlie  a wide  and  irregular  area 
which  is  peripheral  to  a general  uplift  to  the  west.  A few  minor 
areas  are  isolated  and  do  not  occur  with  outcrops  of  the  lower  beds. 
One  of  these  lies  north  of  Burro  mesa,  another  just  to  the  east  of 
Paint  Gap  hill,  and  still  another  north  of  Laguna  inside  of  the 
“rim-rock”  in  the  Chisos  mountains. 

Cottonwood  Creele  Section. 

Along  Cottonwood  creek  there  is  a nearly  entire  section  of 
this  division.  This  creek  is  a branch  of  Bough  Run,  and  comes 
in  from  the  east  around  the  north  end  of  Burro  mesa,  joining 
Rough  Run  opposite  Dogis  mountain.  At  a place  known  as  Chisos 
Pen  this  creek  has  cut  across  the  long  ridge  that  extends  in  a curve 
north  from  the  north  point  of  Burro  mesa.  The  Terlingua  beds 


The  Geology  of  the  Chisos  Country. 


35 


appear  in  a.  continuous  section  along  this  creek,  extending  from 
the  west  side  of  this  ridge  to  about  a mile  beyond.  They  are 
tilted  to  the  east,  which  brings  the  lower  part  of  the  section  to  the 
surface  about  a mile  out  from  the  ridge.  At  this  point  there  is 
a fault,  and  the  section  comes  to  an  end  just  a little  above  the 
Boquillas.  flags. 

The  lower  one  hundred  feet  consist  of  thin  chalky  ledges  in 
every  way  resembling  the  Austin  chalk.  Some  flat  concretions  of 
impure  limonite  lie  in  the  marly  layers  and  perfectly  resemble  the 
concretions  in  the  Niobrara  chalk  in  West  Kansas.  Fragments  of 
Haplascapha  grandis  Conrad  with  attached  valves  of  Ostrea  con- 
gesta  (Con.)  Hall,  are  frequent.  Inoceramus  undulato-plicatus 
Roem.,  another  Inoceramus  of  large  size,  measuring  nearly  two 
feet  across,  and  an  Ammonites  were  noted.  These  beds  dip  east 
at  angles  of  about  AO °.  They  are  followed  by  some  four  hundred 
feet  of  soft  and  gray  marly  sediments,  in  which  the  same  fossils 
appear,  and  wdiich  dip  at  a higher  angle  in  some  places.  Going 
further  in  the  same  direction  we  find  several  hundred  feet  of  clays, 
in  which  there  are  some  layers  of  large  calcareous  concretions,  some 
thin  sandy  ledges  near  the  top  and  also  a sheet  of  intrusive  rock 
measuring  fifteen  feet  in  thickness.  This  clay  is  overlain  by  a 
sandstone,  ten  feet  in  thickness,  wThich  will  be  classified  as  belong- 
ing to  the  Rattlesnake  beds,  the  next  member  in  the  section.  The 
concretions  which  are  common  about  100  feet  below  this  sandstone 
ledge,  contain  now  and  then  ammonoid  shells,  among  which  Pla- 
centiceras  Whitfieldi  Hyatt  and  Nautilus  dekayi  Meek  were  recog- 
nized. The  clays  just  below  this  layer  of  concretions  are  exten- 
sively exposed  for  a mile  on  each  side  of  the  creek,  where  they 
form  a perfectly  bare  country.  They  were  seen  to  contain  a con- 
siderable amount  of  gypsum,  mostly  in  the  form  of  clear  fragments 
of  selenite  crystals.  Fossils  seemed  to  be  wanting. 

A measurement  of  dips  and  outcrops  makes  the  thickness  of 
this  division  here  not  less  than  1280  feet. 

South  of  Cuesta  Blanca, 

Oh  the  'south  side  of  Cuesta  Blanca  to  the  southwest  of  Study 
Butte  is  another  place  where  the  Terlingua  beds  lie  in  an  entire 
continuous  exposure  suitable  for  measurements.  They  overlie  the 


36 


Bulletin  of  The  University  of  Texas. 


Boquillas  flags  which  form  the  table-like  hill,  called  Cuesta  Blanca, 
and  they  dip  to  the  sonth  at  angles  varying  from  25°  to  50°.  The 
measurement  here  falls  a little  short  of  1200  feet.  The  same 
divisions  may  be  noted  at  this  place  a§  in  the  section  .just  de- 
scribed. The  lower  calcareous  strata  are  succeeded  by  more  pure 
clays,  and  uppermost  these  contain  strata  bearing  large  concre- 
tionary lentils  of  calcareous  material,  some  eighty  feet  below  the 
capping  sandstone  which  begins  the  overlying  formation,  ffap- 
loscapha  grandis  Con.,  Ostrea  congesta  (Con.)  Hall,  and  a large 
Inoceramus  occur  about  400  feet  below  the  top,  and  also  farther 
down  in  the  section. 

To  the  south  from  here  there  is  an  extensive  belt  where  these 
beds  outcrop,  reaching  almost  to  the  Rio  Grande,  south  of  the 
Rattlesnake  mountains.  Exogyra  costata  and  E.  ponderosa  were 
found  on  the  bare  flat  surface  of  a uniformly  tilted  outcrop  of 
the  upper  part  of  this  formation  east  of  Dryden’s  ranch.  Bach 
lies  in  a seam  of  rock  which  is  slightly  indurated,  and  which  could 
be  seen  on  the  ground  as  a straight  narrow  ridge  less  than  a foot 
high,  but  extending  for  half  a mile  or  perhaps  farther.  The  two 
ledges  were  perhaps  100  feet  apart  in  the  section  and  E.  ponderosa 
was  in  the  lower  one  of  the  two.  I estimate  that  these  ledges  lie 
here  about  200  feet  below  the  lowest  ledges  of  sandstone  of  the 
next  division. 

Other  Sections. 

Following  the  Rio  Grande  downward  we  next  encounter  the  for- 
mation about  a mile  and  a half  west  of  Talley’s  ranch,  where  the 
lower  chalky  strata  appear  in  the  east  bank  of  the  creek  which 
comes  into  the  Rio  Grande  from  the  north.  The  exposure  reaches 
out  northeastward  for  at  least  two  miles  and  at  a point  one  and 
two-thirds  miles  to  the  east  of  the  ranch  the  upper  clays  were 
found  to  contain  Exogyra  costata.  All  along  the  west  side  of 
Mariscal  mountain  the  formation  underlies  a belt  varying  from 
one  to  two  miles  in  width,  following  the  base  of  the  ridge.  About 
five  miles  north  of  Talley’s  ranch  and  near  the  U.  S.  bench  mark 
having  the  elevation  of  2217  feet,  a hurried  measurement  was 
made  across  the  outcrop  of  the  greater  part  of  the  formation,  leav- 
ing out  one  or  two  hundred  feet  of  the  lower  beds.  This  measure- 
ment makes  the  vertical  distance  from  the  overlying  sandstone  to 


The  Geology  of  the  Chisos  Country. 


37 


the  indurated  beds  below  about  1100  feet.  Two  ledges  about  fifty 
feet  apart  and  lying  some  two  or  three  hundred  feet  below  the 
top  of  the  formation,  consist  of  a large  and  sometimes  continuous 
layer  of  concretions  of  calcareous  material  mixed  with  some  sand, 
and  these  contain  in  some  places  abundant  specimens  of  large 
ammonoids,  among  which  Schloenbachia  conensis  Conrad  (?) 
seemed  to  be  most  frequent.  North  of  this  place,  where  the  wagon 
road  crosses  the  ridge  formed  by  a sill  following  the  west  side  of 
the  mountain,  the  lower  calcareous  division  of  the  formation  is 
well  exposed  in  the  side  of  the  road  and  exhibits  a number  of 
more  or  less  entire  valves  of  large  Inocerami,  some  of  which  meas- 
ure two  feet  across.  North  from  the  end  of  this  ridge  and  at  the 
foot  of  Talley  mountain,  the  concretionary  layers  in  the  upper 
division  have  reached  an  unusual  development  and  are  well  ex- 
posed. Some  of  the  concretions  measure  four  feet  in  diameter. 
This  same  horizon  was  also  noted  in  the  east  bank  of  Tornillo 
creek  at  a point  midway  between  the  village  of  San  Vincente  and 
Boquillas  postoffice,  and  the  same  ammonoid  occurs  there.  Again 
these  strata  come  into  view  at  Banta  Shutup,  where  the  concre- 
tions consist  of  an  impure  clay  ironstone  and  have  large  shrinkage 
cracks  which  are  often  filled  with  black  calcite.  The  beds  here 
follow  the  west  side  of  the  Stillwell  ranch  intrusive,  dipping  to 
the  west.  They  are  also  uncovered  in  the  valley  around  this  ranch, 
where  Exogyra  ponder osa  was  again  noted  and  likewise  the  zone 
of  large  septariu. 

In  the  valley  which  lies  along  the  west  side  of  the  Carmen 
range,  about  three  miles  east  of  Stillwell’s  ranch,  there  is  a gray 
and  somewhat  indurated  marl  or  soft  limestone  which  belongs  in 
the  lower  part  of  this  division.  It  contains  abundant  specimens 
of  a few  fossils,  one  of  which  I think  is  new.  This  is  a sponge 
which  consists  of  a loaf -shaped  body  of  folded  plates  radiating 
from  a central  point.  It  is  undoubtedly  closely  related  to,  if  not 
identical  with,  the  sponge  already  noted  in  the  horizon  of  the 
Bud  a limestone.  The  pl  ates  contain  a fine  network  of  hexactine 
spicules.  There  was  at  the  same  point  numerous  specimens  of 
Jnoceramus  undulato-plicatns  Roem.,  and  of  Schloenbachia  leonen- 
sis  Conrad.  Other  Inocerami  and  a Baculites  asper  Morton  ( ?) 
were  noted.  The  formation  extends  with  some  interruptions  for 


38 


Bulletin  of  The  University  of  Texas. 


several  miles  in  a belt  northwestward  from  this  place,  passing 
Mnskhog  spring. 

In  the  country  between  the  Corizones  peaks,  Box  spring,  and 
the  Christmas  mountains,  the  Terlingua  marls  and  shales  are  fre- 
quently seen,  and  some  of  the  fossils  noted  at  other  places  were 
observed  here,  but  this  region  is  more  broken  up  by  igneous  in- 
trusions and  'sections  suitable  for  description  are  less  comipon. 

The  Laguna  is  a shallow  depression  which  is  caused  by  the 
presence  of  these  easily  disintegrating  beds,  a half  mile  west  of  the 
highest  peak  in  the  Chisos  mountains.  By  the  folding  to  which 
the  ground  has  been  subjected  here,  the  shales  are  drawn  out 
lengthwise  and  shrunk  in  other  directions  so  as  to  be  greatly  re- 
duced in  thickness.  They  are  likewise  more  indurated  and  more 
fissured  by  joints. 

Topographic  Relations. 

In  their  relation  to  topography  the  Terlingua  beds  invariably 
maintain  the  habit  of  appearing  on  the  lowest  ground.  In  the 
center  of  the  Chisos  mountains,  where  they  have  been  raised  by  a 
high  fold,  they  occupy  valleys  among  the  peaks.  On  the  lowlands 
they  form  the  flattest  and  most  desert-like  stretches  in  the  entire 
region.  They  have  yielded  more  readily  to  erosion  than  any  other 
member  of  the  Upper  Cretaceous  series.  This  is  due  no  less  to 
rapid  weathering,  caused  by  the  weak  coherence  of  the  clastic  ele- 
ments, than  to  the  promptness  with  which  these  are  capable  of  be- 
ing carried  away  by  the  sheet  flood  current,  owing  to  their  small 
size.  The  effectiveness  of  the  sheet  flood  is  increased  by  the  im- 
perviousness of  the  clays,  which  causes  the  rainfall  to  be  shed  as 
from  a roof. 

Organic  Remains. 

The  lower  part  of  the  formation  is  largely  composed  of  organic 
sediments.  The  chalky  beds  consist  almost  entirely  of  a deep  sea 
ooze,  quite  free  from  mechanical  ingredients.  This  ooze  consists 
mostly  of  comminuted  fragments  of  delicate  calcareous  shells  of 
foraminifera,  but  entire  tests  may  always  be  found  bv  proper  crush- 
ing and  washing  of  the  rock.  Fragments  of  the  shells  of  Inocera- 
mus  form  an  appreciable  ingredient  in  some  strata.  Including 
some  four  hundred  feet  in  this  lower  division  of  the  formation,  il 
yielded  the  following  fossils : 


The  Geology  of  the  Chisos  Country. 


39 


1.  Globigerina. 

2.  Textularia. 

3.  Other  foraminifera. 

4.  Sponge. 

5.  Sea  urchin. 

6.  Radiolithes,  like  socialis  D’Orb.  This  fossil  is  smaller  than 
Hippurites  t exanus  Roemer,  which  it  otherwise  somewhat  resem- 
bles. Unlike  the  latter,  it  consists  of  a cluster  of  several  individ- 
uals which  are  grown  together.  It  was  found  in  the  marls  north 
of  Laguna,  west  of  Emory  peak. 

7.  R.  austinensis  Roem.,  found  in  the  lowermost  ledges. 

8.  Haploscapha  grandis  Conrad. 

9.  Inoceramus  undulato-plicatus  Roem.  There  were  several 
smaller  Inocerami,  which  were  not  identified. 

10.  Ostrea  congesta  Conrad. 

11.  Baculites  asper  Morton. 

12.  Ammonoids,  two  undetermined  species. 

In  the  midde  division  of  the  formation  no  fossils  were  observed. 
The  upper  three  or  four  hundred  feet  contain  occasional  arena- 
ceous seams  and  layers  along  which  calcareous  material  has  gath- 
ered into  large  concretions,  which  are  sometimes  separate  and  dis- 
tant and  at  other  times  lie  close  together,  or  even  join,  so  as  to 
form  a continuous  bed.  In  some  of  these  concretions  fossil  eepha- 
lopods  are  quite  plentiful,  but  only  a few  forms  have  been  ob- 
served. Exogyra  has  not  been  noted  in  connection  with  the  con- 
cretions, but  it  seems  to  be  limited  in  this  region  to  certain  indi- 
vidual layers  not  far  from  the  horizon  in  which  these  occur. 

The  forms  noted  in  the  upper  three  hundred  feet  are  as  below: 

1.  Exogyra  costata  Say. 

2.  E.  ponderosa  Roem. 

3.  Nautilus  dehayi  (Mort.)  Meek. 

4.  Placenticeras  Whitfieldi  Hyatt. 

5.  Schloenbachia  leonensis  Conrad  (?). 

Chemical  Composition. 

It  will  be  clear  from  what  has  already  been  said  that  the  com- 
position of  the  three  divisions  in  this  formation  presents  two  ex- 
tremes. The  lower  division  is  a chalk  and  the  upper  part  is  a clay. 


40 


Bulletin  of  The  University  of  Texas. 


An  analysis  of  the  chalk  shows  only  a small  amount  of  clayey  ma- 
terial and  an  analysis  of  the  clay  wonld  no  doubt  show  only  small 
quantities  of  calcareous  material.  Frequently  the  clays  in  the 
upper  part  of  the  section  give  no  response  to  acid.  ISTo  analysis 
has  been  made  of  these  clays  but  the  composition  of  two  specimens 
of  the  chalky  beds  was  found  to  be  as  belowT: 

Analyses  of  Specimens  of  the  Lower  Chalky  Ledges  of  the  Ter- 

lingua  Beds  A 


West  of  Chisos  Pen.  South  of  Ouesta  Blanca. 

Silica 4.54  per  cent.  19.34  per  cent. 

Alumina 0.75  per  cent.  3.00  per  cent: 

Ferric  oxide 1.40  per  cent.  2.16  per  cent. 

Lime 52.04  per  cent.  39.50  per  cent. 

Magnesia 0.24  per  cent.  1.00  per  cent. 

Carbonic  acid 40.70  per  cent.  31.50  per  cent. 

Water  (hygroscopic) . 0.10  per  cent.  .30  per  cent. 

Water  (combined)  . . 0.40  per  cent.  2.00  per  cent. 

Sulphur trace. 

Organic  matter .90  per  cent. 


100.17  per  cent.  99.70  per  cent. 


Correlations. 

The  lower  part  of  the  Terlinqua  beds  clearly  corresponds  to  the 
Austin  chalk  in  the  eastern  part  of  the  State.  The  two  forma- 
tions are  alike  and  in  their  lithological  characters  the  beds  are 
practically  of  the  same  kind.  * There  is  a slightly  greater  admix- 
ture of  clayey  material  in  the  westerp  section  than  in  the  eastern, 
especially  above.  The  middle  clays,  which  are  destitute  of  fossils, 
are  with  equal  certainty  the  equivalent  of  the  Taylor  marls  in  the 
Austin  region,  and  there  is  no  good  reason  why  the  upper  member 
should  not  he  referred  here  also.  But  the  upper  limit  of  the  Tay- 
lor marl  has  so  far  not  been  definitely  made  out  for  the  eastern 
region,  and  until  this  can  he  done  a more  particular  comparison 
would  be  premature.  The  upper  two  hundred  feet  herald  the  com- 
ing change  of  conditions,  by  the  isolated  arenaceous  bands  which  it 

^Analysis  bv  Mr.  O.  H.  Palm. 


The  Geology  of  the  Chisos  Country.  41 

contains,  bnt  its  fossils,  so  far  as  known,  do  not  indicate  any  close 
relationship  to  those  which  occur  in  the  formation  that  lies  above. 

The  Rattlesnake  Beds. 

Up  to  the  end  of  the  Terlingna  period  the  sediments  of  the  Cre- 
taceous age  in  this  region  were  laid  down  in  open  waters  that  ex- 
tended beyond  the  shore  belt,  bnt  at  this  time  the  sea  began  to  be 
encroached  upon  by  the  land,  at  least  so  far  that  its  waters  were 
more  or  less  hemmed  in  by  inlands  and  other  shore  structures. 
These  conditions  prevailed  for  some  time.  The  shore  belt  evi- 
dently slowly  subsided,  and  thus  allowed  littoral  deposits  to  ac- 
cumulate to  a considerable  thickness.  The  resulting  sediments 
are  entirely  different  from  those  of  the  preceding  periods.  They 
consist  of  sandstones,  muddy  and  peaty  clays  and  silts,  now  and 
then  some  limestone,  and  occasionally  some  thin  beds  of  gravel. 
The  littoral  conditions  w^ere  such  that  vegetable  accumulations 
occasionally  .were  laid  down  and  buried  among  other  sedimeuts. 
These  have  been  changed  into  coal.  From  the  nature  of  the  con- 
ditions attendant  upon  the  making  of  these  deposits  they  are 
necessarily  quite  variable  in  composition  and  texture,  and  indi- 
vidual strata  do  not  persist  for  any  great  distance  horizontally. 

The  Zone  of  Change. 

While  there  is  no  difficulty  in  making  out  the  general  line  of 
division  between  the  Terlingua.  and  the  Eattlesnake  formations,  as 
this  is  well  marked  in  a general  way,  still  the  particular  level  of 
the  change  in  each  local  section  must  be  selected  somewhat  arbi- 
trarily owing  to  the  frequent  variations  of  the  beds  in  the  upper 
member.  Usually  the  change  is  ushered  in  by  a thin  stratum  of 
sandstone,  which  may  be  overlain  by  fifty  or  more  feet  of  clays,  of 
the  same  kind  we  find  below  it.  Then  there  may  be  another  ledge 
of  sandstone  with  another  stratum  of  clay.  Or  the  sandstones 
may  be  replaced  by  arenaceous  limestone.  These  beds  may  truly 
be  said  to  make  a zone  of  change,  since  they  lead  up  to  the  variable 
deposits  above.  The  sandy  and  calcareous  layers  are  frequently 
fossil  bearing,  and  their  fauna  is  clearly  related  to  that  above.  Yet 
it  is  somewhat  different.  To  make  close  comparisons  possible  with 


42 


Bulletin  of  Tlie  University  of  Texas. 


the  Cretaceous  strata  elsewhere  by  other  students,  the  fossils  ob- 
served in  the  transition  beds  are  here  listed  separately : 

1.  Coral  (?). 

2.  Bryozoan,  adhering  to  the  outside  of  a cup-shaped  tube, 
probably  from  a coral. 

3.  Gyrodes  sp.  ( ?). 

4.  Natica  sp.  (?). 

5.  Rostellites  texana  Conrad  (?). 

6.  Scurria  sp. 

7.  Spironema  sp. 

8.  V olutomorpha,  like  poderosa  Whitfield. 

9.  Camptonectes  burlingtonensis  Cabb.  (?). 

10.  Tliracia  sp. 

11.  Dentalium  gracile  M.  & H. 

12.  Baculites  sp. 

13.  Lamna,  like  elegans , Agaz. 

'4.  Lamna  texana  Boem. 

15.  Vertebrae  of  fishes. 


Thickness. 

Including  a hundred  feet  of  this  zone  of  change  the  Rattlesnake 
formation  contains  about  600  feet  of  strata,  as  delimited  in  the 
description  which  follows.  It  must  be  stated,  however,  that  the 
division  between  these  beds  and  the  sediments  wrhich  overlie  it 
is  much  less  wrell  defined  than  the  base  of  the  formation.  Xo  at- 
tempt could  be  made  to  definitely  mark  its  upper  limit. 

The  Sandstones  of  the  Rattlesnake  Beds. 

The  sandstones  of  the  Rattlesnake  beds  made  the  most  con- 
spicuous outcrops  in  these  sediments.  Single  beds  measure  as 
much  as  fifty  and  sixty  feet  in  thickness.  While  they  vary  con- 
siderably in  color — which  may  be  white,  gray,  yellow,  brown  and 
even  bluish — they  possess  some  features  which  are  rather  constant. 
They  are,  as  a rule,  quite  free  from  coarse  admixtures.  Pebbles, 
even  of  small  size,  are  quite  unknown.  These  sands  hardly  ever 
contain  any  mica  and  they  are  quite  well  sorted.  In  seven  sam- 
ples which  wrere  specially  examined  with  regard  to  texture,  there 


The  Geology  of  the  Chisos  Country. 


43 


was  only  one  specimen  in  which  the  greater  part  of  the  material 
consisted  of  grains  measuring  a.  half  millimeter  in  diameter;  in 
one  sample  the  maximum  ingredient  consisted  of  grains  measuring 
one-fourth  of  a millimeter;  in  one  of  grains  measuring  about  one- 
sixth  mm. ; in  two,  about  one-eighth ; and  in  one  most  of  the  grains 
were  less  than  one-sixteenth  millimeter  in  diameter.  Compared  with 
other  sandstones  of  the  same  coarseness,  the  grains  are  rather  little 
rounded.  Some  magnetic  grains  may  usually  be  found.  Other- 
wise the  constituent  is  quartz,  of  which  some  is  chert,  especially 
among  the  grains  of  the  largest  sizes.  With  the  coarse  admixtures 
organic  calcareous  fragments  are  sometimes  also  imbedded.  Very 
rarely  these  form  thin  seams  between  finer  strata,  where  the  sort- 
ing has  been  affected  by  strong  currents.  Some  calcareous  ma- 
terial is  quite  generally  present,  and  the  sandstones  grade  into  al- 
most pure  ledges  of  limestone,  which,  however,  are  infrequent. 

The  bedding  is  variable.  In  few  instances  faces  of  ledges  were 
seen  measuring  twenty  feet  and  apparently  without  bedding  seams. 
A's  a rule,  thin  bedding  prevails.  In  thin  ledges  the  seams  are 
usually  straight  and  even.  But  cross  bedding  is  very  general,  and 
fine  ripple-bedded  layers  are  not  uncommon. 

Being  the  most  open  in  texture  of  all  the  sediments  in  this  for- 
mation the  sandstones  have  been  the  highways  of  underground 
water,  and  this  has  often  worked  secondary  changes.  The  most 
frequent  change  is  the  introduction  of  interstitial  calcareous  ma- 
terial, giving  the  rock  greater  hardness  and  toughness.  This  in- 
terstitial material  is  sometimes  a pure  calcite  with  continuous 
cleavage  planes  of  large  size.  The  change  is  very  generally  ac- 
companied by  development  of  concretionary  structures,  also  of  large 
size.  A frequent  form  of  these  is  large  circular  disks,  measur- 
ing sometimes  three  feet  in  diameter,  and  exhibiting  the  original 
bedding  planes  on  the  edges  in  weathering.  They  frequently  split 
along  these  planes  into  thin  plates,  an  inch  or  less  in  thickness,  ex- 
tending across  the  entire  disk.  Spherical  concretions  have  also 
developed,  the  sizes  most  commonly  seen  being  from  two  to  six 
inches  in  diameter. 

In  some  of  the  sandstones,  and  especially  in  the  lower  part  of 
the  formation,  cementing  material  is  present  in  small  quantity 
only,  and  mostly  confined  along  the  joints,  leaving  the  central 
part  of  the  delimited  blocks  soft  and  ready  to  crumble  under  the 


44  Bulletin  of  The  University  of  Texas. 

influence  of  the  weather.  This  gives  rise  to  a habit  of  weather- 
ing which  has  a very  nniqne  appearance  and  develops  some  most 
irregular  cavities  on  the  surface  of  the  stone.  Ultimately  these 
run  together  and  leave  bent  and  irregular  plates  and  cusps  pro- 
jecting out  from  the  face  of  the  ledge,  in  the  most  fantastic  forms 
and  resemblances. 

About  three  hundred  feet  above  the  base  of  this  formation  the 
sandstones  have  some  peculiar  markings,  which  are  the  imprints 
of  the  fucoid  Halymenites.  They  .can  be  most  readily  described 
as  closely  resembling  moulds  that  might  be  made  by  pressing  the 
side  of  a small  ear  of  popcorn  into  sand.  They  usually  appear 
as  semi-cylindric  depressions  on  the  weathered  surface  of  the  sand- 
stone, frequently  bending  and  sometimes  branching,  varying  in 
width  from  three-fourths  of  an  inch  to  nearly  twice  that  width 
and  having  a length  of  sometimes  as  much  as  six  or  eight  inches. 
The  surface  of  the  impressions  is  studded  with  shallow  pits  that 
measure  about  three-sixteenths  of  an  inch  in  diameter  and  that  tend 
to  be'  arranged  in  rows  running  diagonally  across  the  cylindric  de- 
pression. In  cross-section,  these  pits  may  be  seen  to  extend  all 
around  the  cylindric  cavities  and  in  a few  instances  a smooth  and 
straight  central  core  of  sandstone  was  seen  to  protrude  from  one  end 
of  the  otherwise  empty  mould.  In  another  place  the  filling  in 
these  moulds  had  for  some  reason  become  more  indurated  than 
the  matrix  around  it,  and  as  this  weathered  away  the  casts  in  the 
original  form  of  the  fossil  were  left  bare.  In  cross  and  longi- 
tudinal sections  these  exhibited  a smooth  central  core  of  unaltered 
sandstone.  He  vestige  of  organic  material  could  be  found  in  any 
of  the  specimens  examined.  In  the  casts  just  mentioned  the  sand 
was  held  together  by  a dark  ferruginous  cement,  which  was  lack- 
ing in  the  surrounding  more  rapidly  weathering  matrix. 

These  markings  are  so  abundant  in  some  sandstone  ledges  that 
they  may  be  said  to  fill  them.  They  have  been  noted  in  the  coal 
bearing  beds  of  the  Upper  Cretaceous  at  Eagle  Pass,  and  they  are 
reported  from  the  Fox  Hills  sandstone  in  Eastern  Colorado  by  N. 
H.  Darton.  Their  real  nature  must  at  present  be  left  to  con- 
jecture. The  cylinders  usually  lie  in  a horizontal  position.  This, 
as  well  as  their  mode  of  branching,  suggests  that  they  may  be  im- 
pressions of  root-stocks  of  some  water  plant.  Perhaps  they  are 


The  Geology  of  the  Chisos  Country.  45 

the  burrows  of  some  animal  living  in  the  beach  sands  of  this 
period. 

Finer  Sediments. 

The  finer  sediments  which  are  interbedded  with  these  sandstones 
consist  mostly  of  silts  below,  and  of  clay  of  finer  texture  higher 
up.  The  muddy  silts  contain  more  or  less  of  carbonaceous  mate- 
rial, which  is  sometimes  present  in  such  quantities  as  to  make  the 
deposits  appear  perfectly  black.  Occasionally  the  silts  as  well  as 
the  clays  are  marly  and  at  times  we  find  the  calcareous  ingredient 
gathered  into  concretions  with  forms  of  all  degrees  of  regularity. 
At  times  these  form  continuous  layers  many  feet  in  extent  and 
from  four  inches  to  a foot  thick.  These  usually  oxidize  to  black 
impure  limonite,  as  the  .original  concretionary  mass  contains  a 
large  per  cent  of  carbonate  of  iron.  At  other  places  the  concre- 
tions form  perfectly  circular  leaves  and  disks.  The  finer  clays  are 
usually  gray,  but  baking  by  igneous  intrusions  and  also  weathering 
may  have  turned  them  yellow,  brown,  or  red.  Bluish  and  greenish 
tints  are  also  in  evidence.  The  finer  sediments  constitute  the  bulk 
of  the  formation. 


Original  Calcareous  Deposits. 

Original  calcareous  deposits  played  a very  subordinate  part  in 
the  sediments  of  this  period.  But  there  are  a few  ledges  of  lime- 
stones which  consist  of  organic  fragments,  in  the  form  of  ill  as- 
sorted calcareous  sand  and  of  mud  containing  larger  shell  frag- 
ments. Some  of  the  clays  as  well  as  some  of  the  sandstones  also 
contain  a calcareous  ingredient  which  is  present  as  an  original 
constituent. 

Special  Sections. 

A description  of  some  local  sections  will  give  a better  idea  of 
the  character  of  this  formation  as  a whole.  In  the  ridge  which 
extends  north  from  the  north  end  of  Burro  mesa  we  find  the  lower 
part  exposed.  Cottonwood  creek  crosses  this  ridge  almost  exactly 
at  the  latitude  of  29°  20'  N.,  and  at  the  east  foot  of  the  ridge  its 
bed  contains  a tinaha,  near  which  there  is  an  old  camping  place 
known  as  Chisos  Pen.  The  creek  crosses  the  ridge  along  a line  of 
fracture  and  the  section  in  the  ridge  is  considerably  broken  up  and 


46 


Bulletin  of  The  University  of  Texas. 


obscured.  The  beds  dip  to  the  east  from  10°  to  30°  and  are  essen- 
tially  as  follows : 

Section- near  Chisos  Pen. 

Thickness  in  feet. 

13.  Soft  gray  ledges  of  sandstone  containing  occasional  shells 


of  two  species  of  oysters 140 

12.  An  intrusive  sill 20 

11.  Silt  and  clay  with  some  carbonaceous  material 60 

10.  Sandstone,  gray 20 

0.  Sand,  mixed  with  clay  and  carbonaceous  material 40 

8.  Coal ' U 

7.  Silty  mud  or  fire  clay 2 

6.  Sandstone  containing  some  mud  and  in  places  showing 

clear  lamination,  a large  oyster  present 50 

5.  Not  exposed ? 


4.  Sandstone,  irregularty  bedded,  and  weathering  to  a very 
irregularly  cavernous  or  pitted  surface,  and  containing 


large  specimens  of  Inoceramus  cumminsi  Cragin 30 

3.  Clayey  and  muddy  sand ? 

2.  Yellow  hard  sandstone  containing  fish  regains  and  vari- 
ous lamellibranchs 10 

1.  Clays  of  the  Terlingua  beds. 


About  a half  mile  east  of  the  Chisos  Pen  some  ledges  of  soft 
sandstone  rest  on  clays  which  contain  silicified  wood,  and  concre- 
tions of  clay  ironstone,  and  at  one  place  a layer  of  a fine  grained 
white  sediment  which  is  evidently  a tuffaceous  deposit.  These 
ledges  here  lie  in  a horizontal  position,  and  taking  into  account 
the  low  pitch  to  the  east  which  shows  in  the  nearest  exposures  on 
the  west,  the  sandstone  apparently  belongs  to  a horizon  about  two 
hundred  feet  above  the  highest  member  in  the  section  just  given. 
In  the  sandstone  are  moulds  of  various  lamellibranchs,  among 
which  a Leda , a Corbicula,  and  a Cardium  were  noted,  and  it  also 
contains  remains  of  saurians,  fishes,  and  turtles  and  some  silicified 
wood. 

About  one  and  one-half  miles  farther  north  the  same  section  is 
seen  on  the  west  side  of  the  ridge  and  may  be  given  thus: 


The  Geology  of  the  Chisos  Country.  47 

Section  north  of  Chisos  Pen. 

Thickness  in  feet. 

12.  An  igneous  sill 17 

11.  Gray  clay  or  shale 55 

10.  An  igneous  sill 20 

9.  Clay  and  silt  containing  Ostrea,  cf.  glabra  M.  & H 20 

8.  Sandstone 14 

7.  Indurated  sandy  silt 45 

6.  Sandstone  with  Inoceramus  cumminsi  Cragin 10 

5.  Sandy  clay 28 

4.  Sandstone  containing  many  fragments  of  shells 16 

3.  Clay .....! 20 

2.  Hard  yellow  sandstone  with  teeth  of  sharks  and  shells  of 

lamellibranchs 5 

1.  Clays  of  the  Terlingua  beds. 

Again,  a mile  south  of  Chisos  Pen  the  same  beds  on  the  east 
side  of  the  ridge  were  measured  and  described  as  follows: 


Section  south  of  Chisos  Pen. 

Thickness  in  feet. 


12.  Igneous  sill 150 

11.  Sandy  shale 40 

10.  Igneous  sill 10 

9.  Sandstone  with  fragments  of  Ostrea  glabra  M.  & H 42 

8.  Coaly  shale 8 

7.  Sandy  shale 40 

6.  Soft  sandstone  containing  a large  oyster,  probably  Ostrea 

contracta  Conrad 12 

5.  Sandy  shale 40 

4.  Sandstone,  with  Inoceramus  cumminsi  Cragin 12 

3.  Shale 30 

2.  Yellow  hard  sandstone  with  occasional  fish  remains 5 

1.  Terlingua  beds. 


Humber  2 and  3 in  all  these  three  sections  are  the  same,  and 
they  belong  to  what  has  already  been  described  as  the  “transition 
beds.”  They  were  seen  in  many  other  places,  as  in  Rough  run  to 
the  northwest  of  Maverick  mountain,  south  of  the  same  mountain 
near  Dawson  creek,  near  the  junction  of  Rough  run  and  Terlingua 
creek,  in  several  places  between  this  point  and  the  mouth  of  Ter- 


48 


Bulletin  of  The  University  of  Texas. 


lingua  creek,  west  of  Mariscal  mountain,  and  also  on  the  Tornillo 
creek  south  of  Boquillas  postoffice.  At  nearly  all  of  these  places 
the  sandstone  numbered  4,  is  found  lying  right  above,  and  it  almost 
invariably  contains  the  characteristic  fossil  Inoceramus  cumminsi 
Cragin.  This  fossil  is  quite  variable  in  shape  and  size.  It  varies 
from  three  to  four  inches  to  twelve  or  even  fourteen  inches  in 
length,  and  the  larger  forms  are  more  gibbous  than  the  smaller. 
Frequently  the  sandstone  is  profusely  filled  with  these  large  shells 
and  their  imperfect  casts  weather  out  in  abundance.  This  ledge 
frequently  presents  a surface  honeycombed  with  cavernous  hollows 
that  are  separated  by  irregular  plates  consisting  of  indurated  ma- 
terial along  its  joints.  For  three  or  four  hundred  feet  above  this 
sandstone  the  beds  are  made  of  variable  strata  of  clayey  and  sandy 
silts,  in  which  there  are  frequently  some  peaty  seams,  and  occa- 
sionally these  develop  into  seams  of  pure  coal.  This  is  the  coal- 
bearing horizon  of  the  Upper  Cretaceous  in  the  west  part  of  the 
State.  It  is  characterized  principally  by  a lamellibranch  fauna  to 
which  also  belongs  at  least  one  brachiopod  and  some  gasteropods 
and  a turtle.  The  forms  most  frequently  observed  resemble  those 
named  in  the  following  list: 

Lingula  cf.  rauliniana  d?Orb. 

Mytilus  [ ?]  sp. 

Veniella cf.  conradi  Whitfield. 

Mactra  texana  Conr. 

Thracia  gracilis  M.  & H. 

Ostrea  cf.  glabra  M.  & H. 

0.  contract  a Conr. 

Cardium  carolinense  Conr. 

C.  cf.  congestum  Conr. 

Crassatela  cf.  obliquata  Whitfield, 

Fragment  of  the  carapace  of  turtles. 

Impressions  of  plant  (?)  [described  above]. 

The  coal  Is  bituminous  and  has  a fine  texture.  It  is  also  char- 
acterized by  containing  small  irregular  pockets  filled  with  a red- 
dish resin. 

Fossil  Wood. 

Above  these  coal-hearing  strata  the  sediments  become  less  sandy 
and  contain  more  clay  and  more  calcareous  material.  This  is  a 


The  Geology  of  the  Chisos  Country.  49 

horizon  which  almost  everywhere  is  characterized  by  the  presence 
of  silicified  wood,  bones  of  vertebrates,  and  stray  layers  of  tuffa- 
ceons  sediments.  Fragments  of  silicified  wood  are  almost  univer- 
sally present  and  entire  trunks  are  not  infrequent.  Some  trunks 
were  seen  measuring  more  than  three  (in  one  instance  six)  feet  in 
diameter  and  as  much  as  forty  feet  in  length.  The  tree  trunks 
almost  invariably  occur  several  together  and  usually  they  lie  on  or 
under  some  ledge  of  sandstone  or  along  some  particular  layer  in 
the  clays.  In  one  instance  all  the  trunks  belonging  to  one  such 
layer  were  seen  to  be  angiospermous  exogens,  probably  all  of  one 
species.  More  than  two-thirds  of  the  wood  otherwise  shows  the 
tracheid  tissue  of  gymnosperms. 

Vertebrate  Remains. 

Associated  with  these  tree  trunks  are  the  remains  of  some  sau- 
rians.  Many  of  these  are  of  large  size.  Long  bones  and  vertebrae 
are  most  common  and  the  centrum  sometimes  measures  as  much 
as  five  inches  in  diameter.  Fragments  of  the  carapace  of  turtles  * 
are  quite  common.  At  a point  about  a mile  south  of  the  mouth 
of  Dawson  creek  the  vertebrate  remains  were  found  to  be  associated 
with  a Viviparus,  like  V.  raynoldsanus  M.  & H.  The  clay  in 
which  these  vertebrates  most  frequently  occur  usually  contains  a 
number  of  rough  yellowish  concretions,  varying  in  sizes  from  a 
half  to  two  inches  in  diameter. 

Tuffs. 

In  these  clays  we  often  find  layers  of  white  impalpable  volcanic 
tuff  more  or  less  mixed  with  other  sediments.  Usually  some 
fragments  of  bones  may  be  found  near  such  layers.  Instances  of 
this  kind  were  noted  on  the  south  side  of  Dawson  creek,  two  miles 
south  of  Big  Bend  postoffice;  in  the  hills  a half  mile  south  of  Dry- 
den’s  ranch  on  lower  Terlingua  creek;  at  a point  about  four  miles 
west  northwest  of  Talley’s  ranch  on  the  Bio  Grande  south  of  the 
Chisos;  in  the  hills  about  two  miles  east  of  Neville  springs,  and 
on  Cottonw’ood  creek  about  a mile  east  of  Chisos  Pen.  The  tuff 
beds  are  usually  only  two  or  three  feet  thick  and  often  less  than 
this.  They  thin  out,  and  thicken  in  short  distances.  Their  asso- 
ciation with  the  vertebrate  remains  suggest  that  the  eruptions 


50 


Bulletin  of  The  University  of  Texas. 


which  produced  them  may  occasionally  have  been  destructive  to 
life  in  the  coastal  regions  of  the  Cretaceous  sea. 

One  of  the  thickest  beds  of  such  tuff  was  seen  in  a small  sharp- 
pointed  butte,  which  rises  from  the  plain  about  two  miles  east  of 
Chisos  Pen.  The  section  which  is  exposed  on  all  the  sides  of  this 
butte  is  as  below  : 

Thickness  in  feet. 

4.  Soft  gray  sandstone 20 

3.  A brecciated  mixture  of  sand,  gravel,  and  lumps  of  clay 
and  sandstone  measuring  as  much  as  a foot  in  diameter. 
Many  of  the  clay  lumps  are  from  a white  tuff  composing 

the  next  number  below 30 

2.  A white  compact  fine  clay  which  largely  consists  of  an  im- 
palpable volcanic  dust.  It  breaks  into  angular  chips  and 
was  seen  to  contain  a fragment  of  a bone  and  a piece  of 
pumice.  This  stratum  is  separated  by  a sharply  marked 


line  from  the  number  below 38 

1.  Dark  shaly  clay  with  sandy  layers.  Some  silicified  wood 
noted 60 


On  the  southwest  end  of  Slickroek  mountain  the  clays  and  sand- 
stones of  this  division  are  overlain  by  similar  tuff  beds  and  brec- 
ciated mixtures,  measuring  some  seventy-five  feet  in  thickness 
The  change  from  the  clays  to  the  toffs  has  the  appearance  of  an 
unconformity.  The  sharp  line  of  contact  between  numbers  1 and 
2 in  the  section  given  above  suggests  at  any  rate  a great  change  in 
conditions.  It  may  very  well  be  that  at  both  these  places  an  ap- 
parent unconformity  might  have  been  produced  by  currents  in  the 
bottom  of  waters  where  the  volcanic  'sediments  fell.  Such  cur- 
rents may  very  well  have  been  strong  enough  to  tear  up  and  re- 
arrange loose  volcanic  mud,  especially  as  the  eruptions  were  prob- 
ably not  far  distant. 

i^bout  two  miles  west  of  Ash  spring  and  at  a place  where  an 
arroyo  approaches  the  south  base  of  a low  cuesta,  there  is  a layer 
of  yellow  limestone  underlying  a tough  sandstone  which  belongs  to 
these  clays.  The  calcareous  layer  is  only  some  twenty  inches  thick, 
but  it  has  a very  peculiar  brecciated  appearance,  as  if  the  material 
had  been  broken  up  and  twisted  or  kneaded  in  some  way  before 
hardening  into  solid  rock.  A similar  layer  was  noted  in  the  same 
beds  a mile  east  of  the  Grapevine  hills  on  the  east  side  of  the 


The  Geology  of  the  Chisos  Country. 


51 


Chisos.  This  rock  seems  to  be  peculiar  to  the  upper  part  of  the 
Rattlesnake  beds,  and  may  help  in  tracing  their  extent. 

Topographic  Features. 

The  Rattlesnake  beds  have  been  named  from  the  Rattlesnake 
mountain,  which  is  a dissected  low  dome  on  the  east  side  of  lower 
Terlingua  creek  and  about  four  miles  south  of  Big  Bend  po'stoffice. 
These  beds  are  exposed  in  an  almost  continuous  but  irregular  belt 
encircling  the  Chisos  mountains,  and  it  varies  in  width  from  less 
than  a mile  to  several  miles.  It  is  a belt  of  greater  relief  than  we 
find  where  the  Terlingua  marls  constitute  the  bed  rock.  In  the 
Rattlesnake  mountains  it  presents  its  most  rugged  appearance  to- 
pographically. In  most  places  we  find  these  beds  more  or  less 
tilted  and  the  exposed  edges  of  the  sandstones  form  long  and  nar- 
row ridges,  seldom  more  than  a hundred  feet  high,  and  mostly 
less  than  this.  The  effect  of  this  unequal  resistance  to  erosion  by 
different  strata  is  frequently  increased  by  the  fact  that  igneous 
rock  sills  have  been  injected  between  them,  and  these  protect  still 
more  effectually  the  sandstones.  This  is  the  case  in  the  Rattle- 
snake mountain,  in  Dogi’s  mountain,  in  Slickroek  mountain,  in 
Chili  Cortaj  and  Talley  mountains,  and  in  the  Grapevine  hills. 
Where  the  strata  lie  in  a horizontal  position  the  sandstones  some- 
times cap  low  buttes  and  mesas  and  we  find  a few  of  these  along 
the  Tornillo  creek  west  of  McKinney  springs  and  also  on  the  flats 
west  of  Oak  canon. 

Fossils. 


Fossils  are  usually  well  preserved  in  the  clays  and  limestones. 
In  the  sandstones  they  occur  mostly  as  moulds,  the  original  sub- 
stance of  the  shells  having  been  removed  by  solution.  In  such 
cases  identification  is  difficult  and  uncertain.  A list  is  appended 
which  includes  all  the  forms  believed  to  have  been  noted  in  the 
formation,  including  the  “transition”  beds. 


Plants. 


1.  Impressions  of  the  fucoid  Halymenites. 

2.  Silicified  wood.  Fragments  and  entire  trunks  of  trees  lie 
in  the  sandstones  and  in  the  clays.  About  fifty  out  of  seventy 


52 


Bulletin  of  The  Univ&rsity  of  Texas. 


specimens  from  different  localities  are  gymnosperms.  The  other 
twenty  exhibit  the  tracheary  tissue  of  exogenous  angiosperms. 

Invertebrates. 

3.  Bryozoan,  a small  fragment  attached  to  a probable  coral. 

4.  Coral  ? 

5.  Lingula , cf.  rauliniana  d’Orb.  In  a mud  layer,  six  feet 
under  the  coal  seam  in  Cottonwood  creek. 

6.  Ostrea  cf.  contracta  Conrad.  This  is  a large  oyster  of  an 
elongated  form.  One  specimen  measured  twenty-seven  inches  in 
height  and  seven  inches  in  width.  'The  hinge  in  one  measured 
nearly  five  inches  from  side  to  side,  seven  inches  in  height  and 
was  an  inch  thick. 

7.  0.  elegantula  Newb. 

8.  0.  cf.  veleniana  Con.  This  is  a species  which  also  resem- 
bles 0.  prudentia  White,  0.  Wyomingensis  Meek,  and  0.  coalvillen- 
sis  Meek.  It  is  the  most  common  of  all  oysters  in  these  beds  and 
varies  in  form  as  well  as  in  size.  It  occurs  mostly  in  carbonaceous, 
muddy  clays. 

9.  0.  subtrigonalis  Ev.  and  Shum. 

10.  0.  cf.  tecticostata  Gabb. 

11.  Lima  sp. 

12.  Inoceramus  cumminsi  Cragin.  Always  in  sandstones  in 
the  lower  part  of  the  formation.  Usually  much  larger  than  the 
specimens  described  as  types. 

13.  I.  crispii?  var.  subcomp'essus  M.  & H. 

14.  Mytilus  [ ?]  sp. 

15.  Leda,  sp. 

16.  Crassatela  cf.  obliquata  Whitfield. 

17.  Cardium  carolinense  Conrad  [?]. 

18.  C.  cf.  congestum  Conrad. 

19.  Cardium,  undet.  A species  very  common  in  some  sand- 
stones. 

20.  Corbicula  cytheriformis  M.  & H. 

21.  Corbicula , sp. 

22.  Cyprina. 

23.  Veniella  cf.  conradi  Whitfield. 

24.  Veniella,  sp. 


The  Geology  of  the  Chisos  Country. 


53 


25.  Thracia  gracilis  M.  & H. 

26.  Mactra  texana  Conrad  [ ?] . 

27.  Undetermined  lamellibranchs. 

28.  Buccinopsis  parryi  Conrad  [?]. 

29.  Viviparus  cf.  raynoldsanus  M.  & H.  This  species  occurs 
in  association  with  saurian  remains  north  of  the  Rattlesnake  moun- 
tains. 

30.  Undetermined  gasteropods. 

31.  Baculites  grandis  [?]  Meek.  Only  two  rather  poorly  pre- 
served specimens  were  observed  in  one  single  locality : in  the  hills 
west  of  Dryden’s  ranch. 

V ertebrates. 

[Determined  by  Dr.  S.  W.  Williston.] 

32.  Claosaurus  [several  limb  bones  and  several  vertebrae]. 

33.  Ceratopsid?  [Lower  extremity  of  a tibia.] 

34.  Dryptosaurus  [several  teeth]. 

35.  Crocodile  [not  previously  known]. 

36.  Several  turtles  [fresh  water  types]. 

Below  I append  a letter  from  Prof.  S.  W.  Williston  of  the  Uni- 
versity of  Chicago,  who  kindly  undertook  to  examine  the  verte- 
brate fossils  from  the  Rattlesnake  beds. 

Chicago,  January  30,  1905. 

Dear  Professor  Udden:  I am  not  yet  sure  of  the  determina- 
tion of  some  of  your  specimens.  There  are  a number  of  limb  bones 
and  vertebrae  of  a moderate-sized  herbivorous  dinosaur,  which 
clearly  belong  in  the  genus  Claosaurus,  with  solid  limb  bones.  A 
lower  extremity  of  a large  tibia  I believe  to  belong  with  the  Cera- 
topsia,  but  I find  no  other  indications  of  that  group.  The  hone  is 
solid  and  hence  can  not  belong  with  the  Hadrosauridae ; it  is  either 
a Ceratopsid  or  a very  large  Claosaur  of  a new  form.  I find  a 
number  of  carnivorous  teeth,  quite  like  those  so  common  in  the 
Laramie  and  referable  to  Dryptosaurus — but  they  are  not  of  much 
value  for  correlation.  The  large  crocodile  is  new  to  science,  and 
hence  of  no  value  in  correlation.  The  turtles  also  are  of  fresh- 
water type,  but  their  determination  is  impossible  until  the  material 
is  restored.  So  far  then : the  fossils  are  of  the  land  and  fresh- 


54 


Bulletin  of  The  University  of  Texas. 


water  Cretaceous,  and  would  have  been  referred  until  recently  un- 
hesitatingly to  the  Laramie  or  Judith  river.  But,  these  forms  also 
occur  in  the  Belly  River  deposits,  which  are  nowT  known  to  be  con- 
temporary with  the  Ft.  Pierre. 

HoAvever,  I have  very  little  hesitation  in  saying  that  the  deposits 
are  from  near  the  top  of  the  Cretaceous,  though  the  collection  lacks 
not  a few  things  characteristic  of  the  Laramie  and  Judith  River 
that  T should  expect  to  find  in  it,  and  has  an  undue  preponderance 
of  Claosaurus-likei  dinosaurs,  which  are  relatively  rare  in  the 
Laramie  or  Judith  River. 

I am  inclined  to  believe  that  you  have  either  a new  vertebrate 
horizon,  or  some  very  decidedly  new  facies  of  a known  horizon 
which  will  repay  thorough  investigation. 

Very  truly  yours, 

S.  W.  WlLLISTON. 


Correlation. 


Whatever  will  prove  to  be  the  equivalents  of  the  Rattlesnake 
beds  in  the  Rocky  mountain  section  farther  north,  T believe  that 
they  are  certainly  identical  with  the  coal-bearing  horizon  at  Eagle 
Pass,  and  also  with  the  coal-bearing  beds  in  El  Paso  county. 

The  Tornillo  Clays. 

Sandstones  become  less  and  less  important  from  the  middle  part 
of  the  Rattlesnake  beds  upward.  The  highest  part  of  the  forma- 
tion consists  largely  of  clays  alone.  Parallel  with  this  change  is  a 
disappearance  of  nearly  all  of  its  characteristic  fossils.  There  are 
at  least  some  six  hundred  feet  of  such  barren  clays  resting  on  the 
Rattlesnake  formation,  but  they  can  not  be  separated  from  it  by 
any  sharp  limit.  As  these  upper  sediments  are  well  exposed  for 
many  miles  along  Tornillo  creek,  they  will  here  be  called  the 
Tornillo  clays. 

Characteristic  Coloring. 

These  clays  may  usually  be  recognized  by  the  peculiar  sombre 
colors:  gray,  dull  olive  green,  dull  blue,  dull  red,  dull  yellow,  dull 
purple,  dirty  brown,  and  at  times  black  and  white,  only  now  and 
then  heightened  by  the  oxidizing  influence  of  heated  intrusives  to 


The  Geology  of  the  Chisos  Country . 


55 


brighter  shades  of  red  and  yellow.  Their  stratification  is  ren- 
dered conspicuous  chiefly  by  these  color s,  which  alternate  promis- 
cuously in  different  layers,  and  sometimes  also  change  in  the  same 
stratum.  On  the  sides  of  many  bluffs  sapped  by  arroyos  and  creeks 
the  different  strata  usually  appear  as  color  bands  varying  in  width 
from  one  to  twenty  or  thirty  feet.  Otherwise  the  stratification  is 
not  very  apparent.  All  is  a fine  clay  throughout,  with  only  now 
and  then  some  thin  lentils  of  sandstone.  These  sandstone  ledges 
are  nearly  always  characterized  by  great  toughness  due  to  cement- 
ing material,  which  makes  them  hard  to  break. 

M echanical  Composition. 

The  clays  have  a very  fine  texture.  They  contain  a large  ad- 
mixture of  exceedingly  fine  particles,  less  than  one  sixty-fourth  of 
a millimeter  in  diameter.  This  renders  them  quite  impervious  to 
water.  But  they  also  contain  a small  ingredient  of  sand  grains, 
some  of  which  are  magnetic  iron.  Their  clastic  elements  are  really 
not  as  well  sorted  as  the  sediments  in  most  extra-littoral  waters. 
They  resemble  in  this  respect  more  such  clays  as  are  laid  down  in 
hemmed-in  waters,  where  currents  are  absent  and  where  all  the 
elements  in  suspension  are  thrown  dowm.  The  small  quantity  of 
sand  which  is  mixed  in  the  clay  as  w^ell  as  the  sand  which  makes 
up  the  stony  layers,  consists  of  grains  that  are  rather  more  angular 
than  usual.  This  fact  no  less  than  the  presence  of  magnetite 
grains  and  the  copious  ferruginous  ingredient  which  causes  the 
coloring,  suggests  that  a large  part  of  the  sediments  have  been 
derived  from  disintegrated  terranes  of  volcanic  origin. 

Calcareous  material  is  scarce.  Usually  these  clays  do  not  effer- 
vesce on  the  application  of  acid.  But  occasionally  a stratum  of 
lime  rock  may  be  found,  and  it  is  apt  to  be  associated  either  with 
sand  or  gravel  and  mixed  with  carbonate  of  iron.  North  of  Nugent 
peak  a stratum  of  this  kind  was  seen,  which  consisted  of  a mass  of 
lenticular  grains  of  carbonate  of  lime  and  iron,  cemented  into  a 
hard  rock,  in  which  there  were  also  sdme  quartz  pebbles.  It  had 
the  appearance  of  a conglomerate  of  calcareous  lumps.  These 
measured  from  one  to  four  millimeters  in  diameter.  Some  had 
the  appearance  of  being  original  in  the  rock,  while  others  were 
more  like  true  subsequent  concretions.  The  largest  had  central 


56 


Bulletin  of  The  Univorsity  of  Texas. 


radiating  shrinkage  cracks  filled  with  calcite.  This  stratum  be- 
longs in  the  upper  part  of  the  formation.  About  a mile  east  of 
the  Grapevine  hills  a stratum  near  the  base  of  these  clays  contains 
a lentil  of  sandstone,  twenty  feet  thick,  which  is  conglomeritic  at 
intervals.  The  pebbles  in  this  conglomerate  are  well  worn  and 
round,  some  measuring  an  inch  in  diameter.  Among  the  pebbles 
in  this  conglomerate,  rocks  of  different  kinds  were  represented 
about  as  follows: 

Limestone  of  Lower  Cretaceous  age]  ....  65  per  cent. 


Chert 23  per  cent. 

Silicified  wood 5 per  cent. 

Quartzite 3 per  cent. 

Worn  calcareous  concretions 2 per  cent. 

Shale 1 per  cent. 

Marble 1 per  cent. 


Chemical  Composition. 

The  variations  in  color,  which  these  beds  exhibit  is  no  doubt  due 
to  their  high  percentage  of  iron.  Some  of  this  iron  is  in  the  form 
of  black  magnetic  grains.  From  this  circumstance  and  from  their 
rather  large  content  of  potash  and  soda  it  may  be  inferred  that 
these  clays  consist  of  the  finest  debris  resulting  from  the  disinte- 
gration of  volcanic  rocks.  Below  is  an  analysis  of  a dark  clay 
from  the  hills  on  the  east  side  of  Rough  run,  about  two  miles  east 
of  Dogis  mountain.* 


Analysis  of  Tornillo  Clay. 


Silica 64.14 

Alumina 18.81 

Ferric  oxide 6.05 

Lime 74 

Magnesia 30 

Potash 1.28 

Soda - 58 

Water  [hygroscopic] 6.00 

Water  [combined] 2.50 


O.  H.  Palm,  analyst. 


100.40 


The  Geology  of  the  Chisos  Country.  57 

Concretions. 

Almost  at  every  place  where  the  clay  is  subjected  to  rapid 
erosion  its  surface  is  covered  by  small  concretions  of  carbonate  of 
lime  and  of  iron,  which  are  left  behind  as  the  loose  clay  is  washed 
away.  These  concretions  are  usually  of  small  size  ranging  from 
that  of  a pea  to  that  of  an  apple.  They  have  a rough  exterior  and 
are  mostly  of  a dark  or  yellow  color.  Sometimes  they  show 
shrinkage  cracks  inside.  In  a few  places,  as  south  of  the  mouth 
of  Dawson  creek,  there  are  large  concretionary  nests  of  yellow  crys- 
tals of  calcite,  which  have  a radial  arrangement.  These  nests 
measured  from  six  to  ten  inches  in  diameter. 

Weathering  and  Creeping. 

The  Tornillo  clays  weather  in  a most  singular  fashion.  It  has 
already  been  noted  that  they  are  so  fine  in  texture  as  to  be  quite 
impervious  to  water.  Inversely  they  will  not  yield  enough  moist- 
ure to  enable  plants  to  grow,  except  where  their  surface  has  been 
mixed  with  or  covered  by  some  land  drift.  When  rain  falls  the 
surface  of  the  bare  clay  swells  up  into  an  exceedingly  sticky  mud, 
which  renders  the  land  practically  impassable  to  man  and  beast. 
Pools  of  water  will  stand  on  the  ground  after  heavy  showers  and 
they  will  evaporate  away  by  heat  and  sunshine  while  only  a small 
part  of  the  moisture  filters  into  the  clay.  When  the  ground  dries, 
the  clay  shrinks  and  cracks  extensively,  but  as  the  moisture  only 
affects  the  upper  one  or  two  feet,  the  cracks  are  limited  to  the 
same  depth.  The  clay  retains  the  moisture  with  such  tenacity  that 
the  outer  layer  of  a moist  lump  will  warp  and  break  off  while  the 
kernel  is  yet  somewhat  plastic.  As  a result,  the  drying  clay  breaks 
up  into  irregular  angular  hard  lumps,  less  than  an  inch  in  di- 
ameter. These  cover  the  unweathered  strata  beneath  to  depths  of 
from  one  to  three  feet  on  hills  and  slopes  where  the  clay  is  bare. 
They  are  hard  and  tough,  sometimes  wholly  'separate  from  each 
other,  and  sometimes  partly  adhering.  With  every  rain  the  process 
is  repeated,  the  lumps  swell  up  and  are  again  dried  and  warped. 
The  swelling  as  well  as  the  warping  produces  a small  creeping 
motion  among  the  clay  lumps,  small  in  extent  but  evidently  power- 
ful. On  slopes,  gravity  aids  all  movements  in  a downward  direc- 


58  Bulletin  of  The  Univarsity  of  Texas. 

tion  and  counteracts  all  other  movements.  In  the  long  run  the 
accumulated  effect  of  this  influence  results  in  a motion  in  the 
direction  of  the  slope.  The  whole  bed  of  clay  lumps  thus  creeps 
forward  like  a glacier.  The  movement  is  evidently  very  slow,  but 
many  of  the  clay  hills  show  unmistakable  indications  of  its  reality 
in  their  rounded  flowing  contours.  Such  conditions  are  especially 
frequent  in  the  hills  along  Rough  run  northeast  of  Maverick  moun- 
tain, along  Tornillo  creek,  northeast  of  Paint  Gap  hill,  and  in  the 
country  to  the  north  and  east  of  Rattlesnake  mountain.  They 
seem  to  be  the  result  of  some  nicely  balanced  adjustment  between 
clay  texture  on  the  one  hand  and  particular  climatic  conditions  on 
the  other,  and  we  must  regard  this  creeping  as  a very  exceptional 
phenomenon  in  nature.  On  Dawson  creek,  south  of  Maverick 
mountain,  I found  a place  where  the  front  end  of  such  a creeping 
body  of  clay  lumps  had  embraced  a dead  mesquite  tree  and  was 
still  holding  this  in  an  upright  position,  after  the  base  of  the  clay 
cliff  had  been  washed  away  by  the  creek.  It  will  be  understood 
from  this  that  the  moving  body  of  the  clay  consisted  of  a tangle 
of  lumps  of  such  shape  and  consistency  that  they  adhered  suffi- 
ciently to  hang  together,  when  dry,  without  support.  Where  two 
such  creeping  bodies  of  clay  come  down  from  opposite  sides  of  a 
small  gully  they  will  sometimes  bridge  the  gully  and  allow  the 
water  to  escape  beneath  them.  Miniature  low  natural  bridges 
formed  in  this  way  were  noted  north  of  the  Rattlesnake  mountains, 
along  the  margins  of  some  of  these  “clay  creeps.”  At  other  places 
small  caves  have  evidently  been  formed  in  a similar  manner. 

Geographical  Distribution. 


In  a general  way  the  Tornillo  clays  may  be  said  to  underlie  a 
circular  belt  of  land  just  inside  that  of  the  Rattlesnake  beds,  sur- 
rounding the  Chisos  mountains.  Its  most  typical  outcrops  are  to 
be  found  along  the  east  bank  of  Rough  run,  south  and  east  of 
Dogis  mountain,  where  they  weather  into  the  peculiar  rounded  con- 
tours which  have  just  been  described.  At  a point  northwest  of 
Chisos  pen  a fresh  cut  in  the  left  bank  of  the  creek  the  bedding 
indicated,  clearly,  an  unconformity.  Some  of  the  layers  had  been 
cut  away  and  the  hollow  was  filled  with  the  same  clav  or  with  a 
clay  identical  in  appearance.  The  structure  was  too  high  up  in  the 


The  Geology  of  the  Chisos  Country. 


59 


bank  to  be  accounted  for  by  the  action  of  the  present  stream.  It 
appeared  like  an  unconformity  in  the  original  bedding.  No  sim- 
ilar feature  having  been  observed  elsewhere,  it  .does  not  seem  likely 
that  there  is  any  general  unconformity  at  this  horizon. 

The  clays  underlie  much  of  the  country  west  of  Burro  mesa. 
South  of  Dawson  creek,  on  a line  nearly  south  of  the  west  end  of 
Maverick  mountain,  some  streaks  of  tuff  are  imbedded,  and  the 
uppermost  part  of  the  section  exhibits  two  seams  of  peaty  material 
about  thirty  feet  apart,  each  of  which  is  overlain  by  a layer  of  sand- 
stone from  two  to  four  feet  thick.  Again,  we  find  these  clays  to 
the  northeast  and  to  the  east  of  Rattlesnake  mountain,  and  along 
Willow  creek.  North  and  northwest  from  Reed's  camp  they  are 
seen  in  most  arroyos  and  also  over  part  of  the  flat  country  between 
this  place  and  Lindsey’s  quicksilver  claim.  They  constitute  the 
bed  rock  over  large  areas  between  and  around  Talley  mountain  and 
Chili  Cortal  mountain,  and  around  the  Dugout  wells.  From  here 
northward  they  underlie  some  later  gravels,  appearing  north  of 
these  in  a belt  which  extends  north  to  the  Tornillo  creek,  on  the 
east  side  of  the  Grapevine  hills.  In  the  valley  of  this  creek  they 
frequently  contain  plates  of  gypsum  with  a fibrous  structure.  From 
this  point  the  outcrops  follow  the  'south  side  of  Tornillo  creek  west- 
ward and  appear  last  on  Onion  flat. 

Physiographic  Character. 

From  the  above  account  it  is  clear  that  the  Tornillo  clays  in- 
variably occupy  the  widest  stretches  of  the  graded  plains  in  the 
Chisos  country.  The  reason  for  this  is  clear.  The  clays  are  soft 
and  yield  promptly  to  the  agents  of  erosion.  Transporting  cur- 
rents most  easily  hold  them  in  suspension.  No  stream  in  this  en- 
tire region  carries  water  so  tenaciously  muddy  as  does  the  Tor- 
nillo. Intrusives  seem  to  have  avoided  these  clays,  and  there  are 
but  few  igneous  sills  present  to  break  the  monotony  of  the  even 
plains  to  which  they  have  been  reduced. 

Thickness. 

It  was  not  practicable  to  satisfactorily  make  out  the  thickness  of 
Tornillo  clays.  About  a mile  and  a half  above  the  mouth  of  Daw- 
son creek  the  combined  thickness  of  these  clays  and  Rattlesnake 


60  Bulletin  of  The  University  of  Texas. 

beds  is  not  far  from  1400  feet.  In  Ash  canon  on  the  northwest 
flank  of  the  Chisos  mountains  the  dip  and  the  outcrop  of  the  clays 
alone  indicate  that  they  measure  at  least  800  feet.  But  the  ex- 
posures here  are  not  continuous,  and  there  may  be  some  faulting. 
On  the  eakt  side  of  the  Chisos  there  seems  to  be  more  of  these 
clays.  West  of  the  eruptives  which  surround  Still  well’s  ranch  and 
about  four  miles  north  of  Banta  shutup,  there  is  a continuous  ex- 
posure of  the  tilted  edges  of  the  Rattlesnake  beds  and  the  'Tornillo 
clays,  which  dip  from  15°  to  25°  to  the  west  for  a horizontal  dis- 
tance in  the  same  direction  of  nearly  a mile.  This  indicates  a 
thickness  of  the  two  formations  of  at  least  1500  feet.  But  the 
upper  limit  of  the  Tornillo  clays  does  not  appear.  Six  or  seven 
hundred  feet  seems  a very  conservative  estimate  for  this  formation. 
Possibly  it  is  nearer  to  a thousand  feet. 

Fossils. 

Very  few  organic  remains  were  observed.  Silicified  wood  is  oc- 
casionally present  in  the  lower  strata.  About  one-half  mile  north- 
east of  Rock  hut,  or  about  two  miles  nothwest  of  Oak  spring,  frag- 
ments of  some  large  saurian  bones  with  very  coarse  texture  were 
noted  in  an  exposure  of  a varicolored  clay.  A little  distance  from 
this  place  a ledge  of  sandstone  in  the  same  beds  was  indented  on 
its  upper  surface  with  curving  grooves  resembling  tracks  made  by 
fresh-water  clams.  They  were  certainly  marks  of  some  organic 
origin.  These  are  the  only  indications  noted  of  the  existence  of 
life  during  the  Tornillo  epoch.  There  can  hardly  be  any  doubt 
that  plants  and  animals  were  less  abundant  in  the  seas  where 
these  clays  were  laid  down  than  in  the  waters  depositing  the  Rat- 
tlesnake beds.  Nevertheless,  it  is  believed  that  a more  thorough 
search  in  the  dark  clays  may  result  in  the  discovery  of  more  fossils. 

The  Chisos  Beds. 

The  uppermost  and  latest  great  formation  in  tfiis  region  consists 
of  a heavy  deposit  of  tuffaceous  sediments,  which  I shall  call  the 
Chisos  beds,  as  they  form  by  far  the  greater  part  of  the  sediments 
seen  in  the  Chisos  mountains. 

In  their  make-up  the  Chisos  beds  are  quite  unique.  They  are 


The  Geology  of  the  Chisos  Country. 


61 


everywhere  clearly  stratified  in  thin  and  well-defined  ledges  and 
layers.  These  measure  from  an  inch  to  one  and  two  feet  in  thick- 
ness, and  persist  for  long  distances.  The  exposures  appear  some- 
what like  those  of  stratified  limestones.  But  the  color  is  usually 
of  a brighter  hue  than  is  common  in  limestones.  The  great  bulk 
of  the  strata  consist  of  a bluish  gray  or  white  stratified  rock,  which 
lies  in  even,  thoroughly  consolidated  ledges.  Among  these  there 
are  occasional  layers  of  clay  and  sandstone,  and  even  thin  layers  of 
conglomerate.  The  sandy  layers  often  show  cross-bedding  and 
occasionally  also  ripple  marks,  and  the  sand  is  quite  well  sorted. 

The  Typical  Rock. 

Examining  the  texture  more  closely  we  find  that  the  typical 
rock  appears  almost  structureless  to  the  naked  eye.  With  a hand 
lens  one  can  distinguish  scattered  grains  of  quartz,  slightly 
rounded,  imbedded  in  an  obscurely  fragmental  mass,  which  con- 
tains small  cavities  or  vesicles  filled  with  various  minerals,  evi- 
dently of  secondary  origin,  such  as  crystals  of  quartz,  amorphous 
quartz,  or  a greenish  silicate  resembling  serpentine,  or  more  rarely 
crystalline  calcite.  Under  the  microscope  small  specks  of  this 
greenish  mineral  are  seen  quite  frequently  throughout  the  rock. 
The  sand  grains  sometimes  show  round  contours,  evidently  due  to 
wear,  and  again  they  exhibit  original  crystal  faces.  Worn  frag- 
ments of  feldspar  may  also  be  observed,  and  there  are  even  now 
and  then  minute  grains  of  magnetite  and  more  or  less  well  defined 
specks  of  limonitic  material.  The  mass  or  matrix  of  the  rock  is 
usually  too  fine  in  texture  to  enable  us  to  recognize  the  individual 
minerals  of  which  it  is  composed,  even  with  the  microscope.  But 
it  is  essentially  siliceous,  for  it  gives  no  response  to  acid. 

By  crushing  the  rock  and  washing  out  the  sand  we  may  find  the 
same  minerals : quartz  in  rounded  and  angular  grains,  seldom  ex- 
ceeding one-fifth  millimeter  in  diameter,  sometimes  of  a very  clear 
variety  and  sometimes  opalescent:  feldspar  in  clearly  worn  frag- 
ments or  of  unabraded  crystals;  magnetite  and  limonitic  '>rains,  the 
latter  evidently  a secondary  product  from  the  former.  Very  rarely 
we  may  discover  fragments  of  scoriaceous  glass. 

In  chemical  composition  these  sediments  are  somewhat  variable 
owing  to  secondary  changes  by  which  they  have  been  affected.  A 


62 


Bulletin  of  The  University  of  Texas. 


specimen  which  was  taken  from  the  exposures  in  the  hills  south  of 
Ward’s  spring,  where  the  beds  appear  to  be  comparatively  little 
altered,  was  submitted  to  Mr.  0.  H.  Palm  for  analysis.  His  report 
is  as  below : 

Analysis  of  the  Chisos  Tuffs. 


Silica 53.42 

Alumina N 9.17 

Ferric  oxide 2.33 

Lime 17.15 

Magnesia SO 

Carbonic  acid 16.00 

Water  [hygroscopic] 22 

Water  [combined] 1.02 

Sulphur trace. 


99.51 

Frequently  these  ledges  have  been  considerably  altered,  even 
when  not  much  changed  in  their  superficial  appearance.  Minute 
crystals  of  quartz,  rather  long  and  slender,  have  formed  throughout 
the  fine  clastic  matrix.  Small  angular  spaces  of  irregular  shape 
have  been  filled  with  chaleedonie  quartz,  or  with  minerals  re- 
sembling serpentine  and  limonite.  Or  entire  ledges  have  been  af- 
fected by  solution  to  such  an  extent  that  large  irregular  cavities 
have  been  formed,  and  filled  with  calcite.  In  such  ledges  no  mag- 
netic sand  is  present,  but  it  seems  to  be  replaced  by  the  limonitic 
material. 

The  Sandstones. 

The  sandstones,  which  constitute  less  than  one-fifth  of  these 
deposits,  are  seldom  very  heavy  but  consist  of  mostly  isolated  ledges 
a foot  or  so  in  thickness,  and  these  rarely  are  more  than  two  or 
three  in  succession.  The  grains  are  largely  angular  quartz,  aver- 
aging perhaps  one-fifth  millimeter  in  diameter.  But  well  rounded 
grains  occur  also.  Feldspar  and  magnetite  grains  are  present. 
The  interstices  between  the  grains  are  filled  either  with  some  fine 
tuffaeeous  material  or  by  calcite  or  by  a siliceous  deposit,  evidently 
brought  by  the  ground  water.  In  one  specimen  taken  south  of 


The  Geology  of  the  Chisos  Country. 


63 


Ward  spring  magnetite  grains  were  plentiful  along  some  thin 
seams  following  a slanting  and  curving  'seam  through  the  ledge. 

The  Clays. 

Clayey  strata  are  most  frequent  and  heaviest  in  the  lower  part 
of  the  formation  where  they  resemble  the  Tornillo  clays.  They 
have  a chocolate  color  and  contain  small  dark  calcareous  concre- 
tions. Higher  up  they  become  thinner  and  acquire  a dark  dirty 
gray  color.  On  weathering,  the  higher  shaly  strata  break  up  into 
cubical  pieces;  and  concretions,  when  present,  are  apt  to  be  larger 
and  to  have  a more  smooth  surface  than  in  the  lower  clays,  i^ear 
Government  spring  some  clay  seams  in  these  beds  are  deep  black 


from  the  presence  of  organic  material.  An  analysis  of  a sample 
from  a typical  layer  of  clay  from  the  exposures  west  of  the  Chisos 
mountains  is  as  below : 

Silica 72.64 

Alumina 12.11 

Ferric  oxide 3.45 

Lime 1.46 

Magnesia 25 

Potash 1.40 

Soda 1.70 

Water  [lrygroscopic] 2.60 

Water  [combined] 4.60 


100.21 

The  Conglomerates. 

Conglomeritic  seams  are  sometimes  present,  especially  in  the 
middle  and  upper  part  of  these  sediments.  But  they  consti- 
tute an  insignificant  fraction  of  the  whole.  Some  of  the  conglom- 
eritic seams  are  associated  with  the  sandy  ledges,  and  their  pebbles, 
which  consist  mostly  of  well-worn  limestone,  average  from  one- 
fourth  or  a half  inch  in  diameter  to  two  or  three  inches.  Else- 
where we  find  well-worn  and  rounded  small  boulders  of  limestone 
as  well  as  igneous  rock  from  three  to  six  inches  in  diameter,  im- 
bedded in  the  stony  ledges  of  finer  texture.  The  limestone  boul- 


64 


Bulletin  of  The  University  of  Texas. 


ders  are  sometimes  partly  silicified  on  the  surface,  so  that  this  is 
protected,  as  it  were,  by  a more  or  less  continuous  crust  of  chert. 

Description  of  Exposures. 

South  of  Oak  Canon  and  about  two  and  a half  miles  to  the 
southeast  of  Rock  Hut  there  is  a high  cliff  of  the  Chisos  beds, 
which  here  lie  in  a nearly  horizontal  position.  Some  600  feet  are 
seen  and  the  exposure  shows  two  igneous  sheets,  one  of  which  is 
about  thirty  feet  thick.  The  other  is  much  thinner  and  in  places 
exhibits  some  vesicles  like  those  in  a lava  flow.  One  sandy  ledge 
contains  rounded  limestone  pebbles.  The  lower  part  of  the  cliff 
exposes  considerable  clay  but  the  upper  part  consists  mostly  of 
sandy  tuffaceous  sediments  with  thin  strata  of  clay. 

South  of  Ward  spring  the  beds  dip  toward  the  south  at  angles 
from  5°  to  35°  and  in  the  ridge  between  this  spring  and  Blue 
creek,  in  all  some  1200  feet  are  in  view,  consisting  of  rapid  alter- 
nations of  the  main  type  of  rock  with  'sandy  and  clayey  layers. 
Some  boulders  and  streaks  of  gravel  occur  in  the  upper  part  of 
this  section.  In  the  Blue  creek  valley  the  formation  rises  in  the 
hills  on  both  sides  to  several  hundred  feet,  and  they  have  been 
carved  into  beautiful  pillars  and  towers  which  adorn  especially  the 
lower  part  of  the  slopes.  Sandy  layers  are  frequently  in  evidence 
and  occasionally  these  contain  pebbles.  In  the  country  which  lies 
south  of  the  mountains  between  this  valley  and  Juniper  creek  the 
Chisos  beds  mostly  constitute  the  bed  rock,  dipping  usually  at  low 
angles.  In  the  bed  of  the  arroyo  which  runs  to  the  east  about  two 
miles  south  of  Lower  Juniper  Spring,  sandy  ledges  are  rather  fre- 
quent. The  more  fine-grained  ledges  have  here  been  altered  by 
the  ground  water  to  such  an  extent  that  they  are  sometimes  diffi- 
cult to  recognize.  Calcareous  material  and  quartz  has  been  gath- 
ered into  pockets  and  concretions  which  stud  the  surface  of  many 
of  the  ledges  in  odd  forms.  The  upper  part  of  Juniper  valley  is 
eroded  from  the  strata  of  this  formation,  and  they  form  the  lower 
slopes  of  Crow  mountain.  At  the  base  of  the  easternmost  spur  of 
this  mountain  a ledge  of  a coarse  and  well-worn  conglomerate  was 
interbedded  between  the  white  siliceous  strata.  Rummel  peak  and 
Nugent  mountain  consist  of  the  same  strata  capped  by  igneous 
flows.  An  estimate  of  their  thickness  as  shown  in  the  east  slope 


The  Geology  of  the  Chisos  Country. 


65 


of  Rummel  peak  makes  them  at  least  2000  feet  and  possibly  2500 
feet  at  this  place.  On  the  north  side  of  the  mountain  they  appear 
in  the  lower  part  of  the  slopes  and  cover  the  territory  between 
Lone  mountain  and  Panther  spring  on  the  east,  and  Moss  well 
and  Government  spring  to  the  west.  In  brief,  we  may  hence  say 
that  the  Chisos  beds  underlie  the  east  half  of  the  mountains,  and 
extend  in  a narrow  crescentic  belt  around  them  on  their  west  side. 
They  also  appear  in  several  places  in  the  country  southwest  of  the 
mountain,  so  far  out  as  south  of  the  Mule  Ear  peaks. 

Limits  and  Thickness. 

The  dividing  plane  between  the  Tornillo  clays  and  the  Chisos 
beds  it  was  not  found  practicable  to  accurately  fix.  Judging  from 
appearances  there  is  a gradual  change  in  two  hundred  feet,  from 
variously  colored  clays  through  yellow  and  gray  clays  and  to  white 
tuffaceous  sands  and  stratified  tuffs.  This  horizon  of  change  may 
be  seen  two  miles  south  of  Oak  spring  and  also  near  the  east  base 
of  Nugent  mountain.  At  both  of  these  places  some  irregularities 
in  dips  were  noted,  but  it  could  not  be  satisfactorily  ascertained 
whether  these  were  due  to  an  unconformity  or  to  some  orogenic 
displacements.  The  evidence  seemed  to  favor  the  latter  view. 
The  upper  limit  of  the  Chisos  beds  is  still  more  uncertain.  Near 
Oak  spring  and  Ward  spring  the  upper  part  of  the  formation  has 
been  removed  by  erosion.  West  of  the  Lower  Juniper  spring  the 
Chisos  beds  are  overlain  by  an  igneous  sheet,  which  lies  in  a hori- 
zontal position,  while  the  strata  below  dip  to  the  east.  In  Rummel 
peak  the  sediments  seem  to  contain  some  lava  flows  farthest  up  in 
the  section,  but  there  was  no  opportunity  to  make  this  point  cer- 
tain. The  general  statement  seems  alone  warranted  at  the  present 
time  that  the  Chisos  beds  consist  of  at  least  two  thousand  feet  of 
strata  consisting  of  highly  tuffaceous  sediments. 

Topography. 

The  Chisos  tuffs  are  more  resistant  to  destructive  agencies  than 
the  underlying  Tornillo  clays  and  they  weather  less  promptly. 
Gullies  and  arroyos  which  drain  their  surface  are  yet  far  from  hav- 
ing reached  the  limit  of  their  transporting  power,  and  the  greater 


66  Bulletin  of  The  University  of  Texas. 

part  of  their  energy  is  spent  in  the  corrosion  of  their  channels. 
And  so  these  tntfs  have  a very  broken  topography,  their  surface 
rising  and  falling  in  ridges  and  slopes  that  often  curve  up  into 
vertical  cliffs  on  the  outer  sides  of  the  Chisos  mountains.  The 
area  of  their  outcrop  is  the  most  picturesque  part  of  the  entire 
landscape. 

Life  and  Age. 

No  fossil  remains  of  any  kind  were  found  in  the  various  phases 
of  the  Chisos  beds.  But  the  considerable  quantity  of  lime  present 
in  some  of  the  strata  and  the  rare  occurrence  of  some  dark  car- 
bonaceous clay  seams  indicates  a probability  of  the  existence  of 
such  remains  in  some  ledges.  Under  the  circumstances  a deter- 
mination of  their  age  is  not  possible,  and  their  reference  to  the 
Cretaceous  is  only  provisional  and  tentative.  But  it  is  probably 
correct.  There  is  no  clear  evidence  of  the  existence  of  an  uncon- 
formity with  the  clays  below,  but  there  appears  to  be  a gradual 
transition  or  change  from  the  lower  to  the  higher  beds.  This  change 
consists  chiefly  in  a gradual  increase  of  the  volcanic  debris  upward. 

The  Crown  Conglomerate. 

In  the  base  of  Crown  peak  at  the  east  end  of  Green  gulch  the 
uppermost  sediments  consist  of  three  conglomerates,  which  are  in- 
terbedded  with  strata  resembling  the  typical  Chisos  beds.  These 
conglomerates  measure  each  some  ten  or  twenty  feet  in  thickness, 
and  a great  many  of  the  pebbles  and  boulders  are  well  rounded. 
Most  of  them  consist  of  various  kinds  of  lavas  and  trap  rocks,  but 
limestones  are  also  well  represented  and  some  of  these  contain 
fossils  of  Lower  Cretaceous  age.  The  strata  dip  here  about  45° 
to  the  north  northeast. 

Conglomerates  of  the  same  kind  are  frequently  seen  in  large 
blocks  which  have  come  down  on  the  slopes  of  the  hills  on  the  east 
side  of  Oak  creek  north  of  Oak  spring.  We  also  find  them  in  Ash 
canon  to  the  west  of  the  “Rim  Rock.”  At  these  places  the  greater 
part  of  the  pebbles  and  boulders  in  the  conglomerate  consist  of 
limestone.  All  are  thoroughly  worn  and  rounded,  even  the  boulders 
which  measure  eight  or  ten  inches  in  diameter.  Carboniferous  fos- 


The  Geology  of  the  Chisos  Country. 


67 


sils  were  noted  in  some  of  the  boulders,  but  the  Lower  Cretaceous 
limestones  are  also  represented. 

Again,  this  conglomerate  was  seen  at  Burro  spring  on  the  west 
side  of  Burro  mesa,  where  it  dips  to  the  east  in  the  hill  north  of 
the  spring  and  is  it  least  forty  feet  thick.  All  the  boulders  are 
well  worn  and  some  measure  a foot  in  diameter.  Probably  the 
greater  number  are  limestone.  Here,  as  at  the  other  places,  the 
limestone  boulders  have  been  silicified  or  sometimes  otherwise  al- 
tered to  a depth  of  about  one-fourth  of  an  inch  from  their  surface. 

Boulders  and  pebbles  of  this  kind  are  seen  in  several  places 
along  the  road  between  Castillon  peak  and  Reed’s  camp,  and  these 
must  have  come  from  exposures  of  'similar  gravels  in  the  region 
between  this  road  and  the  mountains. 

From  the  fact  that  these  gravels  are  interbedded  with  sediments 
quite  identical  in  appearance  with  the  Chisos  beds,  and  since  they 
overlie  these  and  have  the  same  dips,  we  must  suppose  that  both 
are  of  the  same  age,  and  that  the  Crown  gravels  represent  the  latest 
episode  in  the  Chisos  period.  The  further  fact  that  the  Chisos 
beds  have  occasional  layers  of  the  same  kinds  of  gravels,  contain- 
ing rounded  pebbles  of  limestone  and  of  igneous  rocks,  emphasizes 
this  supposition.  The  gravels  were  probably  made  during  the  time 
of  the  last  emergence  of  the  land  from  the  Cretaceous  sea. 

The  Burro  Gravels  and  Tuffs. 

Some  gravels,  breccias,  sandstones,  and  tuffs  are  of  a later  but 
unknown  age.  Such  are  the  sediments  which  directly  underlie  the 
lava  flows  that  cap  the  Burro  mesa,  and  which  were  noted  under  a 
lava  sheet  about  a mile  and  a half  southeast  of  the  mouth  of  Daw- 
son creek.  The  east  escarpment  of  Burro  mesa  exposes  near  Rock 
Hut  several  strata  of  this  kind,  which  dip  to  the  south  at  quite 
high  angles.  In  the  arroyos  that  drain  the  mesa  west  of  this 
place,  there  are  heavy  beds  of  sandy  tuffs  associated  with  the  lavas. 
In  the  hills  southeast  of  the  mouth  of  Dawson  creek,  at  the  locality 
already  mentioned,  a well-worn  conglomerate  is  seen  filling  a small 
arroyo  or  'stream  bed,  cut  in  tuffs  and  again  covered  with  tuffs. 
Upward  and  outward  on  both  sides  of  the  buried  dimunitive  chan- 
nel the  conglomerate  runs  into  the  sand.  In  the  base  of  the  west 
side  of  Crown  mountain  the  edges  of  the  dripping  strata  of  the 


68  Bulletin  of  The  University  of  Texas. 

Crown  conglomerate  are  bnried  under  tuffs  which  underlie  hori- 
zontally the  igneous  cap  of  the  mountain.  Other  conglomerates 
and  sandy  tuffs  of  this  class  were  noted  in  the  hill  which  rises  to 
the  southwest  from  Ward  spring  and  in  Burro  mesa  along  Smug- 
gler’s canon. 

All  of  these  deposits  are  closely  associated  with  lava  flows  and 
some  are  apparently  interbedded  with  them.  Their  general  field 
appearance  is  quite  different  from  that  of  the  Crown  conglomerate 
and  of  the  Chisos  beds.  They  are  less  regularly  bedded  and  they 
vary  much  more  in  composition  and  in  texture.  The  tuffs  are  fre- 
quently yellow  or  even  red  and  sometimes  filled  with  concretionary 
structures  entirely  unlike  anything  seen  in  the  lower  beds,  from 
which  they  are  separated  by  an  unconformity.  If  we  regard  the 
igneous  rocks  as  of  Tertiary  age,  it  follows  that  the  interbedded 
sediments  should  be  of  this  age  also. 

The  Dugout  Clays  and  Gravels. 

On  the  west  side  of  Tornillo  creek  and  northwest  of  Boquillas 
there  is  an  oval  area  about  twelve  miles  long  and  four  miles  wide 
with  its  longer  axis  extending  from  northwest  to  the  southeast, 
where  the  sediments  of  the  Cretaceous  age  are  covered  by  from 
one  to  three  hundred  feet  of  clays,  silts,  sands,  and  gravels  of  a 
reddish  color.  The  finer  sediments  constitute  the  greater  bulk  of 
the  formation,  as  one  sees  it  along  the  road  to  the  Dugout  wells 
from  Boquillas.  In  Bice’s  canon,  a few  miles  to  the  north,  there 
are  mostly  gravels.  The  formation  rises  in  a flat  ridge,  which 
extends  northwestward  from  the  Dugout  wells.  Its  finer  sedi- 
ments resemble  the  Equus  beds  of  the  Great-  Plains  and  also  the 
Pleistocene  river  silts  in  the  valley  of  the  Bio  Grande.  No  fossils 
were  found,  but  as  the  formation  rests  on  a topography  which  has 
been  developed  long  after  the  cessation  of  volcanic  activities  in  this 
region,  it  must  be  of  quite  recent  age  as  compared  with  the  igneous 
rocks  of  the  Chisos  mountains.  But  it  is  no  doubt  older  than  the 
alluvial  deposits  in  the  valley  of  the  Bio  Grande,  for  it  lies  at  a 
higher  level  and  has  itself  suffered  extensive  erosion.  Possibly  the 
Dugout  clays  and  gravels  are  an  old  alluvial  drift  laid  down  by 
Tornillo  creek. 


The  Geology  of  the  Chisos  Country.  69 

Recent  Alluvium  and  Land  Drift. 

The  alluvial  deposits  consist  of  clay,  sand,  and  gravel  that  cover 
the  lower  flats  of  the  streams  and  at  times  rise  into  level  branches 
on  the  sides  of  the  valleys.  It  is  nowhere  very  deep.  Along  the 
Rio  Grande  it  is  usually  a loose  sandy  silt,  but  there  are  also  finer 
clays  of  a darker  and  brownish  color.  These  are  deepest  just  above 
the  canons  which  the  river  is  cutting  through  the  mountain  ridges 
it  traverses,'  and  at  points  where  tributaries  come  in  from  either 
side.  At  the  former  places  the  alluvium  is  silty  and  has  evidently 
been  laid  down  during  great  floods,  when  the  water  has  backed  up 
against  the  narrow  canons  and  has  stood  as  high  as  seventy  feet 
above  low  water,  or  even  higher.  Near  the  junctions  of  tributaries 
with  the  main  stream  the  alluvium  is  usually  gravel,  mostly 
brought  down  by  the  smaller  stream.  A considerable  deposit  of 
such  gravel  is  seen  along  the  lower  course  of  Fresnal  creek  near 
Solis  ranch.  As  a rule,  the  alluvial  deposits  are  not  very  heavy, 
indicating  a condition  of  comparatively  active  corrosion. 

The  land  drift,  which  covers  the  greater  part  of  the  plains,  is 
usually  also  thin.  On  Burro  mesa  it  may  reach  a depth  of  two 
hundred  feet  in  some  places,  but  this  is  an  old  drift,  which  is  now 
being  removed  by  erosion  and  which  must  have  been  laid  down  by 
an  earlier  drainage  quite  different  from  the  present.  In  the  ar- 
royos  the  land  drift  consists  of  gravel  and  sand,  which  is  alternately 
laid  up  in  bank’s  and  again  torn  down  and  rolled  by  the  angry 
water  which  fills  their  channels  after  heavy  showers.  Away 
from  these  smaller  streams  it  consists  of  silt  and  sand,  fine  enough 
to  be  carried  along  by  the  sheet  flood,  and  with  this  is  an  admix- 
ture of  pebbles,  which  are  small  enough  to  be  slowly  rolled  by  the 
same  agent.  The  size  of  these  pebbles  seems  to  bear  a pretty  con- 
stant relation  to  the  slope  of  the  plain.  On  the  average  slope  they 
seldom  exceed  an  inch  and  a half  in  diameter.  Sometimes  there 
are  no  pebbles,  and  in  such  places  the  drift  is  apparently  accumu- 
lating. Elsewhere  they  are  so  plentiful  as  to  form  a thin  continu- 
ous pavement,  and  this  usually  occurs  on  slopes  where  erosion  is 
quite  active. 


70  Bulletin  of  The  University  of  Texas. 

5.  The  Igneous  Bocks. 

No  more  than  a mention  of  the  most  important  occurrences  of 
igneous  rocks  in  this  territory  can  be  attempted  in  this  paper.  My 
held  'studies  would  not  warrant  anything  further.  Observations  on 
the  volcanics  were  made  only  so  far  as  they  had  a bearing  on  the 
general  geological  structure.  Notes  on  some  thin  sections  have 
been  furnished  by  Mr.  B.  F.  Hill,  who  kindly  examined  these  and 
made  such  determinations  as  were  possible  on  short  notice.  It  is 
believed,  nevertheless,  that  the  brief  descriptions  based  on  his 
notes  and  on  such  observations  as  were  made  in  the  held,  may  help 
to  give  a general  idea  of  the  nature  of  the  volcanics  in  this  region 
and  may  be  of  some  service  to  future  observers. 

Igneous  rocks  are  frequently  exposed  and  appear  as  deep  in- 
trusives,  as  laecolites,  as  sills,  as  dikes,  as  plugs  and  as  surface 
hows. 

The  Altuda  Granite. 

An  intrusive  granite  boss  lies  uncovered  over  an  area  of  some- 
what more  than  a square  mile,  four  miles  north  of  Mt.  Ord,  south- 
west of  Altuda.  It  is  a moderately  coarse-grained  rock  of  reddish 
gray  color,  and  it  weathers  into  large  blocks,  sometimes  twenty 
feet  in  length.  It  rises  in  a hill  several  hundred  feet  high.  The 
Carboniferous  sediments  which  surround  this  hill  dip  away  from 
it  and  have  evidently  at  one  time  formed  a continuous  cover  over 
the  wThole  area. 

Laccoliths. 

Some  laecolites  of  the  dome-shaped  type  occur  in  the  region 
north  and  east  of  Terlingua.  Maverick  mountain  is  a conspicuous 
instance.  It  is  a mass  of  intrusive  rock  rising  800  feet  from  the 
plain  and  covering  an  oval  tract  extending  two  miles  east  and  west 
and  about  one  mile  north  and  south. 

The  Rosillos  mountains  must  likewise  be  regarded  as  a laccolite 
broadly  oval  in  form  and  with  diameters  measuring  respectively 
about  four  and  six  miles.  This  rock  consists  of  a feldspathic 
porphyry  occasionally  almost  granitic  in  texture.  The  feldspar  is 
largely  orthoclase  and  is  most  copious  in  the  ground  mass,  which 
also  contains  some  magnetite.  Augite  and  mica  are  present  in 


The  Geology  of  the  Chisos  Country. 


71 


visible  crystals.  The  dull  red  tint  which  this  mountain  shows 
when  seen  from  a distance  is  due  to  the  presence  of  large  spheroid 
kernels  in  the  main  rock,  which  are  more  resistant  to  weathering 
than  the  main  mass  and  hence  are  left  covering  the  surface  of  the 
hills.  These  kernels  measure  from  a few  inches  to  several  feet  in 
diameter  and  they  weather  to  a dull  red. 

The  mountains  around  Stillwell's  ranch  north  of  Banta  shutup 
consist  of  another  laccolitic  intrusion  less  regular  in  form  than  that 
just  described,  but  of  a somewhat  similar  rock.  In  a specimen 
taken  from  near  the  ranch  there  was  a decided  porphyritic  struc- 
ture. The  ground  mass  consisted  of  feldspar,  quartz,  pyroxene  and 
magnetite.  The  phoenocrysts  were  mostly  augite.  Olivine  was 
also  noticed.  In  places  this  intrusion  is  cut  by  dike-like  traver- 
sions  of  a lighter  .rock,  apparently  a quartz  diorite.  These  dikes 
run  in  a northwest-southeast  direction  and  are  usually  only  two 
or  three  feet  thick.  In  weathering,  the  main  body  of  the  rock  in 
these  hills  has  the  same  habit  as  that  in  the  Rosillos  mountains. 

The  northeast  projection  of  the  Chisos  mountains  contains  a 
partly  uncovered  laccolite,  which  extends  over  several  square  miles 
in  the  vicinity  of  Panther  springs.  This  consists  of  a porphyritic 
andesite  in  which  the  ground  mass  has  been  extensively  silicified, 
and  whose  phoenocrysts  appear  to  be  chiefly  plagioclase  and  augite. 
The  rock  is  fissured  by  joints  which  run  N.  35°  E.-S.  35°  W.,  and 
which  in  some  places  are  only  an  inch  apart,  thus  cutting  the  rock 
up  into  thin  vertical  slabs. 

A rock  of  nearly  the  same  character  forms  the  laccolitic  in- 
trusives  known  as  the  Corazone  peaks,  but  a specimen  of  the  ma- 
terial in  one  of  these  peaks  from  near  the  Marlay  wells  was  seen 
to  contain  some  orthoclase  crystals  and  some  original  quartz.  One 
of  these  masses  rises  in  a sharp  peak,  which  is  a thousand  feet  high. 

The  largest  intrusive  mass  in  this  entire  region  is  the  so-called 
“rim-rock”  of  the  Chisos  mountains.  This  rises  to  a height  of 
more  than  two  thousand  feet  and  covers  eight  or  ten  square  miles 
of  territory.  A specimen  from  the  main  mass  of  this  intrusive, 
taken  above  the  falls  in  Oak  canon,  may  be  called  a quartz  por- 
phyry, in  which  the  groundmass  as  well  as  the  phoenocrysts  con- 
sist of  quartz  and  feldspar,  with  a preponderance  of  the  former 
mineral  in  the  groundmass.  The  groundmass  was  somewhat  de- 


72  Bulletin  of  The  University  of  Texas. 

composed  and  there,  was  evidence  of  silicification  by  infiltration. 
The  joints  in  the  rock  are  close  together  and  they  have  nearly  the 
same  bearing  here  as  at  Panther  spring,  about  N.  30°  E.-S.  30°  W. 

Sills. 

There  may  be  some  other  igneous  masses  of  a laccolitic  nature, 
but  those  already  mentioned  are  the  most  important.  Igneous 
sills  are  even  more  common.  These  are  thinner  sheets  that  have 
been  injected  as  molten  material  between  the  ledges  of  the  bedded 
rocks  and  which  run  parallel  with  these  ledges,  sometimes  for  many 
miles. 

There  are  several  different  types  of  rock  among  these  sills.  On 
the  east  side  of  Mariscal  mountain  at  a point  about  two  and  one- 
half  miles  west  and  a half  mile  south  of  Solis’  ranch,  there  are 
two  sills  in  the  Boquillas  flags,  each  measuring  about  five  feet  in 
thickness.  They  are  separated  by  about  forty  feet  of  the  flags. 
The  lower  sill  consists  of  a dioritic  rock.  The  upper  one  is  a 
greenish  black  obsidian  which  breaks  into  more  or  less  rectangular 
blocks.  It  is  plainly  intrusive,  but  along  the  contacts  of  the  sheet 
with  the  sediments  there  is  Tiardly  a suggestion  of  change  in  the 
latter  by  baking.  The  obsidian  is  almost  a perfect  glass  except 
for  a few  spherulites,  which  may  be  seen  in  a thin  section,  and  for 
some  few  crystals  of  orthoclase  and  augite,  some  of  which  are  large 
enough  to  be  visible  to  the  unaided  eye. 

The  greater  number  of  sills  consist  of  either  of  two  types  of 
rocks.  They  are  andesitic  or  dioritic.  A sill  of  andesitic  rock 
is  exposed  in  a line  of  hills  extending  to  the  southeast  from  the 
east  side  of  Terlingua  creek  opposite  Dryden’s  ranch.  This  sill 
is  about  seventy  feet  thick.  It  consists  of  greenish  dark  and  heavy 
rock,  of  moderately  fine  texture,  fissured  vertically  by  joints  which 
are  only  two  or  three  inches  apart,  and  weathering  into  a multitude 
of  round  boulders,  from  half  an  inch  to  three  inches  in  diameter. 
In  a thin  section  one  finds  that  it  is  porpyhritic  in  structure,  with 
a very  fine  groundmass.  The  latter  consists  of  pyroxene  [augite?], 
plagioclase,  and  much  magnetite.  The  phoenocrysts  are  augite  and 
plagioclase,  with  a few  crystals  of  mica.  Perhaps  this  rock  may  be 
regarded  as  intermediate  between  andesite  and  basalt.  Other  sheets 
like  this,  or  extensions  of  the  same  sill,  appears  in  several  ridges 


The  Geology  of  the  Ghisos  Country. 


73 


to  the  west,  to  the  south,  and  to  the  east  of  this  place  and  also  in 
the  Rattlesnake  mountains  and  on  the  east  side  of  Willow  creek. 
Another  instance  was  noted  in  the  Chisos  Pen  ridge  and  in  the 
Dogis  mountain.  West  of  the  Chisos  mountains,  sill's  of  this  rock 
have  quite  generally  been  injected  among  the  strata  of  the  Rattle- 
snake beds. 

A much  more  extensive  sill  of  dioritic  rock  follows  the  Boquillas 
flags  on  the  east  side  of  the  mountains.  This  sheet  is  particularly 
conspicuous  on  the  west  side  of  the  Boquillas  ranges  north  of  Still- 
well’s ranch  and  about  three  miles  east  of  Mailbox  tank.  At  a 
point  about  four  miles  east  and  one  mile  north  of  McKinney’s 
spring  this  sill  is  some  fifty  feet  thick.  It  is  a dark,  coarse-grained 
rock  with  nearly  one-half  of  its  bulk  consisting  of  augite.  The 
feldspar  is  prevailingly  orthoclase.  Magnetite  is  abundant,  and 
the  sand  in  the  arroyos  which  cut  the  sheet  is  often  black  from 
•the  abundance  of  its  magnetite  grains.  Pyrite  is  also  present, 
especially  in  the  lower  two  feet  of  the  sheet,  when  freshly  exposed. 
Olivine  was  noted,  and  in  a thin  section  it  was  seen  to  be  changing 
to  serpentine.  Apatite  is  present  in  a sheet  of  the  same  character 
and  in  the  same  beds  high  up  in  the  Chisos  mountains  about  a 
mile  west  of  Emory  peak.  Along  the  west  side  of  Mariscal  moun- 
tain we  have  apparently  the  same  sill,  but  here  it  is  often  consider- 
ably altered.  The  olivine  is  mostly  changed  to  serpentine  and  the 
magnetite  to  hematite  or  limonite.  A sheet  which  covers  a hill 
lying  about  three  miles  northwest  of  Dugout  Wells  and  northeast 
of  Nugent  mountain  consists  of  a more  acidic  rock  which  lacks 
distinct  groundmass  and  contains  much  magnetite.  The  feldspar 
is  mostly  orthoclase  and  there  is  some  quartz.  The  sills  in  Lone 
mountain  and  in  Grapevine  hills  are  of  the  same  nature. 

Bikes. 

Three  specimens  may  be  taken  as  types  of  the  dike-forming 
rocks.  One  is  from  a dike  about  four  miles  east-southeast  of 
Reed’s  camp  on  the  Rio  Grande,  south  of  the  Chisos.  This  is  a 
porphyry  of  dark  color.  The  phoenocrysts  consist  of  a few  but 
large  feldspar  crystals,  mostly  orthoclase,  and  a mineral  resembling 
acamite.  There  are  also  crystals  of  augite.  The  groundmass  is 


74 


Bulletin  of  The  University  of  Texas. 


of  ophitic  texture,  partly  of  the  same  minerals  as  the  larger  crystals, 
but  containing  also  apatite  and  magnetite. 

Another  specimen  is  from  the  system  of  dikes  which  radiate 
toward  the  southeast  from  Christmas  mountains.  This  may  be 
called  a basic  andesite.  It  is  a dark  rock  with  a fine,  almost 
felsitic  groundmass,  consisting  mostly  of  plagioclase  and  magnetite. 
Its  phoenocrysts  are  quite  large  and  consist  of  plagioclase,  augite, 
and  magnetite. 

The  third  specimen  is  from  one  of  three  dikes  near  Ward  spring 
and  may  be  called  a quartz  porphyry.  There  are  several  dikes  of 
this  kind  in  the  foothills  of  the  Chisos  mountains.  Three  cut  the 
Chisos  beds  to  the  southeast  from  Rock  Hut  and  extend  toward 
Ward  spring.  This  specimen  is  a highly  siliceous  rock,  quartz  ap- 
pearing in  the  groundmass  as  well  as  in  the  larger  crystals.  In 
another  specimen  from  the  same  dike  quartz  and  feldspar  was  seen 
to  be  present  in  about  equal  proportion.  Several  dikes  of  the  same 
nature  follow  the  same  trend  to  the  southeast  of  the  mountains. 

Plugs. 

A sharp-pointed  butte  locally  known  as  the  “'black  peak,”  which 
rises  from  the  plain  about  four  miles  northeast  of  Box  spring,  is 
clearly  a remnant  of  a vertical  plug-like  intrusion.  It  may  or  may 
not  have  been  connected  with  a volcanic  vent.  It  is  an  almost 
perfectly  black  porphyry,  which  in  many  respects  resembles  the 
rock  of  the  dioritic  sill  intruded  among  the  Boquillas  flags  east  of 
Tornillo  creek.  It  consists  almost  entirely  of  plagioclase,  mag- 
netite, and  augite,  and  these  minerals  make  up  the  visible  crystals 
as  well  as  in  the  groundmass.  Olivine  is  an  accessory  mineral. 

Surface  Flows. 

In  the  upper  part  of  the  Chisos  beds  there  are  some  igneous 
sheets  which  evidently  are  interbedded  with  the  strata  of  the  for- 
mation. They  may  have  been  flows  on  the  shores  of  the  waters  in 
which  these  beds  were  deposited.  These  sheets  lie  parallel  with 
the  strata  of  the  sediments  and  at  the  same  time  exhibit  vesicular 
structure.  The  heavy  sheet  in  the  high  bench  east  of  Ash  spring 
and  north  of  Oak  spring  appears  to  be  of  this  kind.  A specimen 


The  Geology  of  the  Chisos  Country. 


75 


of  this  rock  was  seen  to  have  a porphyritic  structure  with  some 
very  large  slat-like  crystals  of  plagioclase  in  a fine  grained  and 
even  felsitic  groundmass.  Magnetite  is  present  in  considerable 
quantity  and  there  is  a filling  of  a green  structureless  mineral  not 
unlike  chlorite,  which  lies  in  many  interstices  among  the  other 
minerals,  especially  in  the  basal  part  of  the  sheet.  This  rock  forms 
a sheet  more  than  a hundred  feet  in  thickness,  and  it  extends  as  a 
vertical  cliff  a distance  of  at  least  a half  mile  in  the  side  of  the 
mountain.  Between  Oak  spring  and  Ward  spring  other  thinner 
flows  with  vesicular  structure  lie  in  the  white  ledges  of  the  tuff- 
aceous  sediments. 

The  lava  sheet  which  forms  the  peak  of  Mount  Emery  is  a quartz 
porphyry  with  a matrix  which  is  occasionally  almost  glassy,  ex- 
hibiting spherulites.  The  specimen  examined  had  evidently  been 
silicified  through  secondary  changes.  The  flow  which  surmounts 
the  high  mesa  to  the  southeast  of  this  peak  is  of  the  same  nature. 

There  are  other  flows  of  still  greater  extent  in  Burro  mesa.  In 
the  north  point  of  this  mesa  no  less  than  five  flows  appear  in  the 
north  escarpment,  and  these  dip  to  the  south.  Tuffs  are  inter- 
bedded  with  these,  some  of  which  have  a fine  compact  texture,  while 
others  grade  through  volcanic  sands  into  breccias.  At  a point 
about  a mile  southwest  of  Bock  Hut  one  lava  sheet  appears,  which 
is  only  a few  feet  thick.  Its  upper  surface  is  checked  by  cracks, 
an  inch  wide  and  from  six  to  seven  inches  apart,  and  these  are 
filled  with  volcanic  sand.  In  Smuggler’s  canon  on  the  west  side 
of  this  mesa,  the  sheet  which  is  exposed  in  the  brink  of  the  falls  is 
a hard  felsite,  and  this  rests  on  a soft  and  yellow  volcanic  sand. 
A half  mile  east  of  this  place  some  lavas  contain  occasional  small 
streaks  of  black  obsidian  with  inclosed  large  crystals  of  feldspar. 

Various  other  surface  flows  are  to  be  seen  southwest  of  the 
Chisos  mountains.  One  was  observed  dipping  at  a high  angle  with 
the  sediments  in  the  north  part  of  Green  gulch. 

D.  GEOLOGICAL  STRUCTURE* 

The  country  traversed  lies  between  the  Southern  Pacific  Railroad 
and  the  Rio  Grande  river  in  the  longitude  of  the  Chisos  mountains. 
Only  a few  days  were  spent  in  that  part  of  this  tract  which  lies 
north  of  29°  30'  north  latitude.  But  here  I found  structural  fea- 


76 


Bulletin  of  The  Univ&rsity  of  Texas. 


tnres  of  such  unusual  interest  that  I can  not  pass  them  by  without 
mention.  These  have  been  in  part  described  by  R.  T.  Hill  before 
this.  The  area  as  a whole  is  traversed  diagonally  from  north 
northwest  to  south  southeast  by  the  southernmost  extension  in  the 
United  States  of  the  great  Rocky  mountain  fold.  This  mighty 
system  of  mountains  is  here  represented  by  a nartrow  and  sharp 
and  for  the  most  part  single  fold,  less  than  two  miles  wide.  It  is 
known  at  the  north  as  Altuda  mountain,  at  the  south  as  the  Car- 
men range,  and  between  these  two  as  the  Ord  and  the  Santiago 
ranges.  East  of  the  line  of  these  ranges  the  great  plains  have 
their  beginning,  though  they  here  more  resemble  a plateau.  On 
the  west  side  we  meet  with  the  typical  structure  of  the  Basin  region, 
and  this  is  continued  westward  beyond  the  limits  of  the  State. 
North  of  Altuda  the  structures  representing  the  Rocky  mountains 
flare  out  and  apparently  soon  have  a width  of  many  times  three 
miles.  With  the  beginning  of  the  Carmen  range  on  the  south 
there  is  again  an  abrupt  increase  in  the  width  of  the  continental 
“backbone”  and  this  increase  is  continued  into  Mexico. 

The  question  at  once  suggests  itself : what  caused  this  principal 
crest  of  the  great  mountain  range  to  contract  to  such  insignificant 
dimensions  between  Altuda  and  the  Carmen  range?  I believe  that 
the  right  answer  to  this  question  is  to  be  found  in  the  fact  that 
the  Rocky  mountain  axis  here  crosses,  almost  at  right  angles,  an- 
other and  much  more  ancient  mountain  system  represented  by  the 
Caballos  ridges. 


E.  THE  CABALLOS  RIDGES. 

The  town  of  Marathon  lies  near  the  center  of  a triangular  plain, 
which  extends  for  about  fifty  miles  to  the  northeast  from  the  Ord 
range.  Against  this  range  the  plain  abruptly  terminates  on  the 
west.  On  this  plain  the  Cretaceous  sediments  have  been  entirely 
removed  and  the  ancient  floor,  on  which  these  were  laid  down  and 
which  they  once  covered,  is  now  bared.  This  floor  consists  of 
sharply  folded  and  highly  tilted  strata  of  Palaeozoic  sediments. 
All  around  the  plain  the  slightly  raised  edges  of  later  sediments 
form  a well-marked  escarpment.  The  folds  of  the  older  strata 
trend  in  a northwest-southeast  direction  and  extend  the  whole 
length  of  the  plain.  They  exhibit  such  regularity  and  persistency 


The  Geology  of  the  Chisos  Country . 


77 


of  direction  that  they  have  all  the  appearance  of  being  a small  part 
of  an  extensive  system.  Their  axes  point  on  the  one  hand  straight 
to  the  Solitario  uplift,  which  is  forty  miles  distant  to  the  south- 
west, and  which  exposes  similar  sediments,  underlying  Cretaceous 
strata,  folded  in  nearly  the  -same  trend.  In  the  opposite  direc- 
tion the  folds  point  toward  the  Ouachita  system  of  mountains  in 
Oklahoma.  Prof.  E.  T.  Hill,  who  has  described  these  ridges,*  be- 
lieves that  this  folding  was  produced  contemporaneously  with  the 
great  Appalachian  movements  in  the  eastern  part  of  the  continent. 
His  observations  are  given  in  the  following  words:  “These  (the 
Caballos  ridges)  are  low  ridges  of  Palaeozoic  rocks  rising  from  the 
floor  of  the  Marathon  plain  south  of  the  Comanche  mountains. 
They  are  composed  of  the  degraded  vertical  edges  of  Palaeozoic 
limestones,  shales,  and  cherts  occurring  in  closely  folded,  buckled 
ridges,  trending  northeast  and  southwest.  The  cherts  are  often 
white  in  color  and  over  100  feet  thick,  and  form  the  backbone  of 
long,  low  ridges,  such  as  the  Pena  Colorado,  occupying  the  low  area 
between  the  Santiago  Sierra  on  the  west,  the  Glass  mountains  on 
the  north,  the  Maravillas  Plateau  on  the  south,  and  the  Stockton 
Plateau  on  the  east.  Caballos  Sandia,  a beautiful  drill  of  white 
chert  about  15  miles  south  of  Marathon  is  the  highest  of  these 
summits,  and  rises  about  1000  feet  above  the  plain  to  an  altitude 
of  about  5000  feet. 

“The  Caballos  and  Glass  mountain  are  exposures  of  ancient  post- 
Palaeozoic  structures  of  Appalachian  type  and  age,  which  have 
been  revealed  by  the  erosion  of  the  Cretaceous  sediments  that  prob- 
ably once  embedded  them.” 

The  same  author  says  of  the  Ouachita  mountains**  that  these 
“are  in  general  old  and  represent  the  remnants  of  once  more  lofty 
and  extensive  ranges,  which  have  undergone  degradation  since  early 
Mesozoic  time.”  While  there  is  probably  no  continuity  between  the 
Caballos  folds  and  the  Ouachita  system  there  seems  to  be  good 
ground  for  the  belief  that  they  were  made  at  the  same  time.  The 
Caballos  ridges  were  made  and  cut  down  by  erosion  during  the  time 
which  elapsed  between  some  period  (probably  the  late  Carbonifer- 
ous) of  the  later  half  of  the  Palaeozoic  era  and  the  beginning  of 

*R.  T.  Hill,  Physical  Geography  of  the  Texas  Region,  'U.  S.  Geol.  Surv. 
Topographic  Atlas  of  the  U.  S.,  Folio  3,  p.  4. 

" ''Reference  as  above. 


78 


Bulletin  of  The  University  of  Texas. 


the  Cretaceous  age.  They  no  doubt  furnished  a part  of  the  ma- 
terial in  the  making  of  the  Triassic  and  Jurassic  deposits  else- 
where. 

It  was  after  the  Caballos  ridges  had  been  cut  down  to  almost  a 
peneplain  and  after  this  peneplain  had  been  buried  under  all  but 
the  latest  Cretaceous  sediments,  that  the  movements  began  which 
resulted  in  the  making  of  the  Rocky  mountains.  Folds  and  faults 
were  made,  which  crossed  diagonally  the  axis  of  these  earlier  flex- 
ures. Now  it  seems  clear  that  where  a fold  is  developing  along  an 
axis  which  crosses  a belt  that  has  been  previously  compressed  and 
folded  in  a direction  more  or  less  vertical  to  the  axis  of  the  later 
disturbance,  the  belt  so  affected  will  less  readily  yield  to  the  forces 
at  work  than  the  country  on  either  side.  For  on  either  side  of 
such  a system  of  folds  the  strata  lie  more  nearly  horizontal,  while 
they  stand  edgewise  in  the  folds.  One  can  easily  bend  a block  of 
paper  in  a direction  which  is  vertical  to  the  plane  of  the  sheets, 
but  not  in  a direction  which  is  parallel  with  this.  Rather  than  to 
bend  in  this  direction  the  sheets  will  break.  Between  Altuda  and 
the  Carmen  range  the  rigidity  of  the  folded  Palaeozoic  strata 
caused  them  to  break  in  a single  sharp  dislocation  forming  the 
Santiago  and  the  Ord  ranges,  rather  than  bend  into  several  folds 
or  fissures  by  several  smaller  faults  as  seems  to  have  happened  on 
either  side  of  this  belt  of  the  ancient  mountain  structure. 

F.  OVERLAP  OF  THE  CRETACEOUS. 

Another  structural  feature  due  to  pre- Cretaceous  mountain 
making  in  this  region  is  the  evidence  of  an  overlap  in  the  basal 
strata  of  the  Cretaceous  formation.  The  nature  and  extent  of  the 
overlap  is  not  known,  but  there  is  no  doubt  that  it  exists.  The 
base  of  the  Lower  Cretaceous  at  Altuda  does  not  include  a com 
glomerate  which  is  a pronounced  feature  of  Persimmon  gap  and  at 
the  Cienega  mountain.  The  same  conglomerate  is  also  absent  in 
the  Solitario,  which,  as  already  stated,  lies  on  the  axis  of  the 
Caballos  uplift  but  it  reappears  at  Shatter,  which  is  to  the  west  of 
this.  It  is  evident  that  the  old  mountain  axis  still  stood  somewhat 
higher  than  the  country  on  either  side  of  it,  when  the  Cretaceous 
sea  was  advancing,  and  in  this  way  its  lowermost  sediments  are 
present  on  the  flanks  of  the  uplift  but  not  on  its  highest  elevations. 


The  Geology  of  the  Chisos  Country.  79 

G.  THE  ROCKY  MOUNTAIN  UPLIFT. 

As  already  stated,  the  structure  which  represents  the  Rocky 
mountain  uplift  in  this  region  consists  of  the  Ord,  the  Santiago  and 
the  Carmen  ranges.  In  general  the  line  marked  by  these  ranges  is 
a line  or  belt  of  displacement,  to  the  west  of  which  the  ground  has 
sunk,  and  to  the  east  of  which  it  has  been  elevated.  Farthest  north 
the  displacement  is  most  gentle.  West  of  Altuda  the  limestones  of 
the  Carboniferous  age  and  the  Lower  Cretaceous  limestones  dip  to 
the  west  at  angles  of  most  frequently  about  15°,  until  in  about 
three  miles  they  disappear  under  the  eruptives.  Farther  south  the 
dip  west  of  the  face  of  the  Ord  range  becomes  more  steep  and  prob- 
ably changs  to  a fault.  At  Persimmon  gap  in  the  Santiago  range 
the  main  Lower  Cretaceous  limestone  is  thrown  into  an  abrupt 
fold  with  a nearly  vertical  dip  on  the  west  side  and  a fault  on  the 
east  side  with  the  downthrow  to  the  east.  Evidently  the  range 
was  here  originally  a sharp  anticline,  which  fractured  on  the  east 
side.  On  the  whole,  the  range  here  involves  a displacement,  along 
which  the  terranes  lying  on  the  west  side  have  been  let  down  at 
least  2500  feet  more  than  the  terranes  on  the  east,  while  the  nar- 
row block  which  forms  the  ridge  has  been  elevated  some  1500  feet 
more  than  the  latter.  At  Dog  canon,  which  cuts  across  the  range 
at  a point  about  five  miles  farther  south,  the  range  is  clearly  a 
compressed  fold,  in  which  the  ledges  of  the  Lower  Cretaceous  lime- 
stones now  stand  in  a vertical  position.  The  arch  of  the  fold  has 
been  fractured  and  removed  by  erosion  and  its  base  exhibits  even 
a fan  structure  due  to  lateral  compression.  Eight  miles  south  of 
this  point  the  Carmen  range  begins.  This  is  clearly  a continua- 
tion of  the  dislocation  we  find  in  the  ranges  to  the  north,  but  the 
lateral  thrust  has  here  given  rise  to  a number  of  folds  of  more 
gentle  build.  The  downthrow  to  the  west  is  quite  as  great  but 
less  abrupt,  and  the  faulting  is  more  frequent,  but  nowhere  is  vio- 
lent. This  condition  gives  a hint  as  to  the  location  of  the  pres- 
ent course  of  the  Rio  Grande.  A stream  which  backs  across 
the  lowest  sag  of  a developing  moutain  crest  will  naturally  out- 
strip other  streams  in  capturing  tributaries.  It  has  less  cutting 
to  do  at  the  start,  and  after  a larger  drainage  area  has  once  been 
acquired,  the  greater  volume  of  water  is  likely  to  insure  the  re- 


80 


Bulletin  of  The  University  of  Texas. 


tention  of  the  advantage  gained,  even  if  conditions  at  first  existing 
should  be  reversed,  as  they  are  in  this  case.  At  present  the  Carmen 
range  is  higher  than  the  south  part  of  the  Santiago  range,  and  it  is 
several  times  as  wide,  but  it  must  have  been  the  lowest  part  of  the 
Rocky  mountain  fold  in  this  region  at  the  beginning. 

H.  THE  GREAT  TERLINGUA  FAULT. 

At  the  mouth  of  Terlingua  creek  the  Rio  Grande  crosses  a great 
fault,  which  runs  a course  about  1ST.  36°  W.  for  some  ten  miles 
north  of  the  river  and  at  least  as  far  to  the  south  of  the  river.  At 
this  point  the  total  displacement  is  about  three  thousand  feet.  This 
decreases  as  we  follow  the  fault  northward  and  it  increases  for 
some  distance  when  followed  into  Mexico.  The  fault  is  marked 
by  a high  escarpment,  facing  the  east.  This  is  caused  by  a rise 
of  the  Lower  Cretaceous  limestone  above  the  present  plain  of 
erosion  of  the  more  readily  yielding  clays  and  marls  of  the  Upper 
Cretaceous  sediments.  This  fault  clearly  illustrates  a feature  re- 
cently described  by  Keyes.*  The  friction  against  the  rising  block 
has  been  sufficient  to  abruptly  bend  upward  the  edge  of  the  more 
flexible  strata  on  the  side  of  the  downthrow.  On  the  face  of  the 
great  escarpment  south  of  the  mouth  of  the  Grand  canon  a thin 
block  of  the  limestone  measuring  several  hundred  feet  in  length, 
is  rotated  in  the  plane  of  the  fault  in  such  a manner  as  to  suggest 
that  one  end  of  the  block  has  been  held  down  by  the  sunken  block 
while  the  other  has  been  more  free  to  follow  with  the  great  wall 
from  which  it  has  been  disengaged  by  a fracture. 

I.  THE  SUNKEN  BLOCK. 

If  we  limit  the  Chisos  country  to  the  region  lying  between  the 
ranges  representing  the  Rocky  mountains  on  the  east  and  the  great 
fault  at  the  mouth  of  Terlingua  creek  on  the  west  and  regard  it  as 
extending  from  the  Rio  Grande  river  on  the  ■south  to  the  Corazones 
and  the  Rosillos  mountains  on  the  north,  we  may  say  that  it  covers 
a part  of  a sunken  block,  which  measures  about  thirty-nine  miles 
from  east  to  west  and  which  has  settled  from  four  to  six  thousand 
feet  below  the  level  of  the  terranes  or  either  side. 

^Journal  of  Geology,  Vol.  XIII,  No.  1,  p.  68. 


The  Geology  of  the  Chisos  Country. 


81 


While  this  block  clearly  maintains  the  nature  of  a structural 
unit,  it  has  itself  suffered  extensive  and  in  places  violent  deforma- 
tion by  faulting  and  folding.  These  deformations  are  so  numerous 
that  a detailed  account  of  them  is  out  of  the  question  at  this  time, 
though  there  is  no  doubt  that  it  would  throw  important  light  on 
the  nature  and  the  distribution  in  time  of  the  orogenic  forces 
which  have  been  at  work  in  this  region.  But  it  has  been  necessary 
to  limit  attention  to  a few  of  the  more  salient  features. 

1.  Faults. 

To  the  northeast  of  the  Rattlesnake  mountains  several  nearly 
vertical  faults  cut  the  almost  horizontal  strata  into  narrow  blocks. 
Two  of  these  are  on  either  side  of  an  elevated  block  which  appears 
in  the  north  bank  of  Terlingua  creek  about  a mile  south  of  the 
mouth  of  Dawson  creek.  The  downthrow  on  either  side  is  at  least 
two  hundred  feet.  The  creek  turns  and  follows  the  east  side  of 
this  block  for  nearly  a half  mile  above  this  point.  Both  faults  are 
vertical  and  both  show  the  edges  of  the  strata  on  either  side  turned 
up  against  the  fault  plane  as  by  friction  against  the  raised  walls. 
Their  course  is  about  FT.  22°  W.  and  the  distance  between  them  is 
about  900  feet.  Within  a mile,  as  we  go  east,  are  two  more  faults, 
parallel  with  these.  These  have  the  downthrow  to  the  east  and  in 
the  case  of  the  farther  one  this  amounts  to  several  hundred  feet. 
They  bear  about  N.  14°  W.,  and  apparently  continue  for  several 
miles.  These  four  faults  are  clearly  genetically  related,  lying  in 
one  parallel  system,  and  this  system  continues  for  at  least  a few 
miles  south  and  two  or  three  miles  north. 

The  east  side  of  Burro  mesa  is  another  fault-line,  which  extends 
from  Rock  Hut  to  south  of  Blue  creek  in  a direction  about  N.  5° 
W.,  with  the  downthrow  to  the  west.  This  mesa  is  a block  which 
has  settled  deeper  than  any  other  part  of  the  great  sunken  Chisos 
country,  and  as  a consequence  the  lower  members  of  the  great  lava 
complex  which  once  covered  the  entire  country  has  so  far  escaped 
destruction  by  erosion.  It  has  let  these  down  some  two  thousand 
feet  below  their  former  level  on  either  side,  and  southwest  of  Ward 
spring  they  are  'seen  to  lie  against  the  Chisos  beds.  The  west  side 
of  the  mesa  is  a monocline  where  the  Chisos  beds  rapidly  rise  from 
under,  as  we  proceed  in  that  direction.  Between  the  north  end  of 


82 


Bulletin  of  The  University  of  Texas. 


this  mesa  and  the  south  end  of  the  Christmas  mountain  is  a belt 
where  disturbances  are  frequent.  At  a point  about  three  and  a 
half  miles  south  and  one  mile  east  of  Christmas  spring,  an  arroyo 
locally  known  as  Cottonwood  creek  crosses  a fault  which  runs  a 
course  nearly  due  north  and  south  and  which  has  a downthrow  to 
the  w^est  of  not  far  from  two  thousand  feet.  It  brings  the  Tornillo 
clays  down  to  the  level  of  the  base  of  the  Terlingua  beds,  and  it  can 
be  traced  for  nearly  two  miles,  but  it  seems  to  turn  to  the  west 
farther  north  and  to  become  continuous  with  a great  fault  which 
follows  the  west  side  of  the  Christmas  mountains. 

To  the  southeast  of  the  Chisos  mountains  and  crossing  the  Rio 
Grande  there  are  several  faults  which  bear  nearly  due  north  and 
south.  Thus  between  Mariscal  and  San  Vincente  mountains  there 
is  a'  block  about  one  and  one-fourth  mile  wide,  which  has  been 
dropped  down  from  one  to  two  thousand  feet.  It  is  marked  off  on 
the  west  by  a vertical  faulted  wall  forming  the  east  side  of  Mariscal 
mountain  and  on  the  east  by  a fault  which  crosses  the  river  di- 
rectly east  of  the  Solis  ranch.  Another  fault  crosses  the  river  to 
the  east  of  the  village  of  San  Vincente  and  from  there  it  runs 
almost  due  north,  passing  on  the  west  side  of  the  TJ.  S.  benchmark, 
having  an  elevation  of  1881  feet.  Three  miles  farther  north  it 
passes  just  to  the  east  of  the  benchmark  with  the  elevation  of  1925 
feet.  The  downthrown  in  this  case  is  on  the  west  side,  and  it 
measures  somewhere  from  500  to  1000  feet,  decreasing  to  the  north. 

North  and  northeast  of  the  Chisos  is  a group  of  faults  which 
have  the  same  general  trend  as  those  on  lower  Terlingua  creek. 
One  of  these  is  the  Muskhog  spring  fault.  It  runs  S.  23°  E.  and 
points  straight  in  a continuous  line  with  the  west  line  of  the  first 
fold  of  the  Carmen  range.  At  Muskhog  spring  the  Rattlesnake 
beds  have  been  lowered  by  this  fault  to  the  level  of  the  upper  ledges 
of  the  Boquillas  flags,  against  which  they  now  abut.  The  down- 
throw is  to  the  west.  This  fault  is  one  of  the  best  developed  de- 
formations of  its  kind  in  the  country,  and  it  was  traced  for  several 
miles.  Two  small  abrupt  folds  with  the  same  trend  follow  it  in 
the  vicinity  of  the  spring,  at  a distance  of  some  two  hundred  yards 
to  the  east.  These  folds  are  next  to  another  smaller  fault  which 
brings  up  the  lower  ledges  of  the  Boquillas  flags.  Several  faults 
with  this  trend  cut  the  ground  in  the  mountains  around  the  Still- 


The  Geology  of  the  Chisos  Country.  83 

well  ranch  and  the  Eosillos  mountains  are  rent  by  at  least  three: 
one  to  the  east  and  one  to  the  west  of  Stroud’s  ranch  and  one  in 
the  arroyo  of  Cottonwood  spring.  The  latter,  which  is  rather  a 
fissure,  extends  southeast  across  Tornillo  creek  and  through  the 
Grapevine  hills,  where  it  clearly  is  compound  and  has  let  down  a 
small  block,  cutting  the  sill  which  caps  these  hills. 

It  will  be  noted  that  the  trend  of  the  axes  of  the  displacements 
so  far  described  is  more  or  less  parallel  with  the  sides  of  the  sunken 
Chisos  block.  Some  faults  have  been  noted  which  run  a course 
that  is  vertical  to  this  direction.  At  a point  about  one-half  mile 
north  of  Muskhog  spring,  three  faults,  which  are  not  two  hundred 
feet  apart,  bear  W.  10°  S.-E.  10°  K.  A thin  block  is  raised  some 
three  hundred  feet  between  two  of  these  faults,  and  the  third  one 
*[the  one  farthest  south]  has  the  downthrow  on  the  north  side.  To 
the  southwest  of  McKinney’s  spring  a fault  cuts  across  the  igneous 
sill  at  its  north  end.  An  unusual  form  of  faults  was  seen  on  the 
north  side  of  Cuesta  Blanca.  Here  the  Rattlesnake  beds  have 
been  let  down  to  the  level  of  the  Boquillas  flags  along  a line  which 
is  concave  to  the  north  but  otherwise  has  a general  direction  from 
east  to  west.  On  the  south  side  of  the  Eosillos  mountains  an  east- 
west  fault  runs  about  a mile  ’south  of  Stroud’s  ranch.  But  the 
most  singular  of  all  faulting  in  this  entire  region  was  noted  in 
Grace  canon  about  half  a mile  northeast  from  the  McKinney  and 
Parker  mine.  A block,  about  300  feet  square,  has  been  let  down 
some  500  feet,  so  that  the  Rattlesnake  beds,  which  compose  it,  lie 
against  the  lower  part  of  the  Terlingua  beds.  The  fault  marking 
the  outside  of  the  block  is  distinct  enough  to  be  traced  with  the 
point  of  a knife. 

In  general,  it  is  true  that  the  faults  whose  trend  is  vertical  to 
the  direction  of  the  axes  of  the  principal  dislocations  have  an 
irregular  development.  This  may  be  taken  as  an  indication  that 
the  making  of  some  of  these  was  incidentally  the  result  of  the 
forces  which  produced  the  faults  trending  H.  H.  W.-S.  S.  E. 

2.  Folds. 

But  a very  'small  part  of  the  sunken  Chisos  block,  if  any,  has 
not  been  affected  by  folding.  The  sediments  only  exceptionally 
lie  horizontal.  Even  when  apparently  undisturbed,  they  are  usually 


84:  Bulletin  of  The  University  of  Texas. 

found  to  be  tilted  a few  degrees  in  some  direction.  The  most  fre- 
quent dips  are  from  10°  to  30°.  A detailed  representation  of 
these  deformations  would  show  a number  of  synclines,  anticlines, 
and  monoclines  of  varied  length,  mostly  trending  in  a northwest- 
southeast  direction. 

Among  the  disturbances  whose  trend  is  parallel  with  the  great 
structures  of  this  region  we  find  an  anticline  west  of  the  Rattle- 
snake mountains.  It  is  fully  three  miles  wide.  The  west  side  of 
Burro  mesa  exposes  a belt  of  Upper  Cretaceous  sediments  which 
dip  a little  north  of  east.  Along  a line  that  points  about  10°  east 
of  south  from  Indianola  peak  runs  the  axis  of  another  anticlinal 
fold.  Several  folds  of  unknown  extent  are  in  the  country  between 
the  Chisos  mountains  and  Boquillas  postoffice,  and  all  appear  to 
have  a northwest-southeast  trend.  An  extensive  syncline  with  the 
same  trend  has  its  axis  about  midway  between  McKinney’s  spring 
and  Neville  spring.  Banta  Shutup  is  on  the  southwest  periphery 
of  the  south  half  of  a quaquaversal,  and  this  may  be  the  south 
end  of  an  anticline  extending  northward.  For  three  miles  to  the 
southeast  of  Maverick  mountain  the  dip  is  quite  uniformly  to  the 
south  and  southeast.  The  fault  on  the  east  side  of  Burro  mesa 
marks  an  axis  from  which  the  'strata  dip  away  on  either  side.  This 
anticlinal  feature  of  the  fault  appears  at  a point  west-southwest 
from  Ward  spring  in  the  north  side  of  a high  butte.  A similar 
anticlinal  arrangement  is  seen  in  Cottonwood  creek,  a mile  west 
of  Chisos  Pen.  The  block  on  the  west  side  dips  west  at  a low 
angle,  and  the  block  on  the  east  is  tilted  away  from  the  fault  first 
gently  and  farther  out  more  rapidly. 

Many  folds  trend  in  various  other  directions.  Hayes  ridge, 
which  forms  the  easternmost  spur  of  the  Chisos  mountains,  is  an 
anticline  trending  nearly  east  and  west.  North  and  northeast  of 
Moss  wells  the  Chisos  beds  dips  to  the  north,  and  south  of  Paint 
Gap  hill  the  Rattlesnake  beds  often  pitch  to  the  south.  Changes  in 
direction  and  amount  of  pitch  are  frequent  and  often  rapid.  The 
entire  sunken  block  is  broken,  folded,  and  twisted  in  an  almost 
bewildering  complexity  and  the  manifold  attitudes  of  the  sedimen- 
tary strata  defy  brief  detailed  description. 

If  we  divide  the  great  sunken  block  into  two  halves  by  a straight 
line  extended  midway  between  the  Rocky  mountain  fold  and  the 


The  Geology  of  the  Chisos  Country . 85 

face  of  the  great  Terlingna  escarpment,  and  nearly  parallel  with 
the  latter,  we  will  find  that  this  line  connects  the  three  greatest 
deformations  affecting  the  block.  These  are  the  Christmas  moun 
tains  at  the  north,  the  Chisos  mountain  uplift  in  the  middle  and 
the  Mariscal  mountain  farthest  south. 

3.  Christmas  Mountain  Fold. 

As  seen  from  the  south  end,  this  mountain  is  an  elliptic  dome- 
shaped uplift  a little  more  than  three  miles  long  and  not  quite 
two  miles  wide,  with  the  longer  axis  extending  N.  36°  W.-S.  36° 
E.  The  Lower  Cretaceous  limestones  are  raised  2200  feet  above 
the  surrounding  plain  and  are  lifted  3500  feet  above  their  position 
under  the  Upper  Cretaceous  sediments  on  the  surrounding  plains. 
The  dome  is  fractured  on  the  west  side  by  several  faults  and  has 
here  an  abrupt  slope,  which  in  places  is  a vertical  scarp.  On  all 
sides  it  has  suffered  considerably  by  erosion,  but  the  form  of  the 
dome  is  still  quite  apparent.  In  all  probability  the  uplift  is  due 
to  an  intrusive  mass  which  is  nowhere  exposed  but  lies  concealed 
under  the  folded  limestone  strata  that  yet  forms  the  roof  of  the 
dome.  At  the  south  end  the  principal  fault,  which  is  marked  by  a 
vertical  cliff,  is  associated  with  an  extensive  Assuring  of  the  lime- 
stone, and  the  fissures  are  filled  with  large  quantities  of  calcite. 
These  fissures  have  the  same  trend  as  the  fault. 

4.  The  Mariscal  Mountain  Fold. 

From  the  top  of  Mount  Emery  two  long  mountain  ridges  may 
be  seen  to  the  southeast,  extending  north  across  the  Rio  Grande 
from  Mexico.  One  of  these,  lying  farthest  east,  is  called  the  San 
Vincente  mountain.  It  stops  nearly  short  of  the  international 
boundary  and  the  river  has  just  begun  to  cut  into  its  north  foot- 
hills. The  other  is  known  as  the  Mariscal  mountain  and  this 
pushes  its  north  end  across  the  Rio  Grande  nine  miles  into  Texas. 
The  river  has  cut  a canon  diagonally  across  this  ridge  and  this 
canon  is  1400  feet  deep  at  the  crest  of  the  ridge.  The  ridge  is  an 
unsymmetrical  fold,  whose  west  limb  has  in  places  been  tilted  into 
a vertical  position  and  whose  east  limb  is  much  less  inclined  but  is 
abruptly  faulted  at  its  east  end,  farthest  to  the  south.  Several 


86  Bulletin  of  The  University  of  Texas. 

minor  faults  cut  the  limbs  of  the  fold.  The  ridge  comes  to  a cus- 
pidate point  at  its  north  end.  At  its  widest  place  the  ridge  meas- 
ures about  three  and  a half  miles  across.  It  consists  entirely  of 
the  Lower  Cretaceous  limestones,  but  the  Upper  Cretaceous  beds 
have  also  been  involved  in  the  uplift,  as  their  dips  adjust  them- 
selves to  that  of  the  fold  over  a Jbelt  from  one  to  three  miles  wide 
on  all  sides. 

5.  The  Chisos  Mountains  Fold. 

The  most  violent  of  the  deformations  of  the  great  sunken  block 
is  an  abrupt  fold  caused  by  the  intrusions  of  the  “rim-rock”  of  the 
Chisos,  and  this  may  be  regarded  either  as  an  enormous  plug  or 
as  an  unusually  high  laccolite.  The  evidence  is  inconclusive  in 
regard  to  which  it  most  resembles.  On  the  west  side  the  Chisos 
beds  lie  nearly  horizontal.  As  we  follow  them  up  to  the  great 
igneous  mass,  we  find  them  thrown  into  small  but  abrupt  folds 
close  to  the  vertical  wall  of  the  mountain.  On  the  south  side  the 
same  relation  is  maintained : the  sediments  run  up  abruptly  against 
the  igneous  mass.  But  on  the  east  and  on  the  north  sides  it  is 
different.  Here  the  sediments  have  been  elevated  and  they  rise 
from  the  east  in  the  form  of  a half  arch,  which  bends  over  the  top 
of  the  “rim-rock,”  and  from  the  north  they  rise  along  a sharp 
contact,  which  stands  at  an  angle  of  75°  from  the  horizontal.  This 
attitude  of  the  sedimentary  rocks  on  the  east  and  north  sides  is 
decidedly  like  their  habit  around  laccolitic  intrusions,  but  their 
position  to  the  south  and  west,  where  there  is  no  regular  tilting, 
is  much  more  like  the  behavior  of  stratified  terranes  around  plugs  • 
or  near  dike-like  intrusions.  The  fact  that  this  igneous  mass  has 
been  seen  to  take  on  a brecciated  structure  in  one  or  two  places 
[as  at  a point  about  one  mile  west  of  Laguna]  favors  the  hypoth- 
esis of  a volcanic  vent.  Such  a theory  would  also  explain  the  pres- 
ent physiographic  feature  of  the  mountains,  for  a volcanic  cone 
would  naturally  effectively  retard  the  , erosion  of  the  country  it 
covered  and  cause  this  to  stand  higher  than  the  surrounding  coun- 
try for  a long  time  after  the  cone  itself  were  carried  away  by 
erosion.  But  even  on  this  theory  the  south  “rim-rock”  may  be 
regarded  as  to  some  extent  laccolitic  in  its  nature.  For  it  is  very 
well  known  that  molten  magmas  may  stop  short  of  reaching  the 
top  of  their  vent  and  may  find  their  way  out  far  down  under  the 


The  Geology  of  the  Chisos  Country. 


87 


surface  and  form  intrusive  masses  more  or  less  distant  from  the 
source  of  the  supply.  If  a vent  should  form  over  a sharp  fold  the 
underground  stresses  might  very  well  be  such  as  to  favor  the  ac- 
cumulation of  an  intrusion  near  the  vent  itself  and  close  to  the 
fold,  or  even  within  it.  The  fissure  through  which  the  molten 
material  escaped  may  have  been  under  the  west  side  of  the  “rim- 
rock.”  At  the  south  it  was  then  forced  in  under  the  Middle  Cre- 
taceous sediments  and  these  were  elevated,  but  at  the  north  the 
intrusion  which  forms  the  north  “rim-rock”  apparently  found  an 
outlet  at  a higher  level  and  perhaps  on  the  concave  north  and 
upper  side  of  the  flexure  produced  by  the  uplift  to  the  south.  This 
intrusion,  or  other  intrusive  flows  from  the  same  source,  appear 
to  have  been  forced  far  out  under  the  latest  Cretaceous  sediments 
to  the  north  and  east.  Everything  considered,  this  seems  to  the 
present  writer  the  most  probable  structure  of  the  Chisos  moun- 
tains. The  whole  round  cluster  of  peaks,  including  an  area  ten 
miles  in  diameter,  is  a remnant  pedestal  under  a volcanic  pile, 
protected  from  erosive  destruction  by  the  endurance  of  the  ancient 
cone,  which  once  covered  it  and  is  now  all  but  wholly  removed. 
The  two  “rim-rocks”  and  other  scattered  laccolitic  masses  are  the 
roots  of  the  cone  itself,  or,  to  carry  the  comparison  farther,  they 
are  tuberous  swellings  on  these  roots,  now  more  or  less  uncovered 
by  erosion.  What  we  find  in  the  Chisos  mountains  today  is  then 
the  structure  developed  at  a depth  of  several  thousand  feet  under 
the  surface  of  ancient  lavas.  These  lavas  were  at  least  in  part 
forced  up  through  fissures  in  the  strata  which  are  now  exposed. 

J.  SUMMARY  OF  GEOLOGICAL  HISTORY. 

The  earliest  record  known  is  that  of  an  open  sea,  during  the 
Trenton  age,  on  the  bottom  of  which  sea  calcareous  sediments  and 
fine  mud  were  deposited.  Marine  conditions  continued,  whether 
with  or  without  interruptions  is  not  known,  until  near  the  end  of 
the  Carboniferous  age  or  possibly  somewhat  later.  Then  a dis- 
turbance occurred  which  resulted  in  the  making  of  a folded  moun- 
tain range,  whose  axis  extended  in  a northeast-southwest  direction 
an  unknown  distance  beyond  the  limits  of  the  ridged  plains  now 
surrounding  Marathon.  During  the  time  which  elapsed  after  the 
making  of  these  moutains  and  until  the  beginning  of  the  Cre- 


88 


Bulletin  of  The  University  of  Texas. 


taceou'S  age — essentially  during  the  Triassic  and  the  Jurassic  ages 
— this  mountain  range  was  reduced  almost  to  a peneplain,  so  that 
when  the  Cretaceous  sea  advanced  the  old  folds  did  not  rise  much 
above  the  rest  of  the  land,  probably  not  more  than  a few  hundred 
feet.  The  Cretaceous  sea  endured  for  a long  time.  At  first  on 
open  and  wide  ocean  held  its  sway  and  on  its  bottom  heavy  deposits 
of  calcareous  mud  and  ooze  settled.  In  time  the  waters  became 
less  deep  and  the  sediments  gave  indications  of  a slowly  approach- 
ing shore  line,  as  they  changed  to  marls  and  clays.  Distant  vol- 
canoes on  the  yet  distant  land  occasionally  mingled  their  ashes 
with  the  clays  of  the  sea.  Later  on  the  shore  line  came  near.  Clay 
and  sand  and  sometimes  even  gravels  and  vegetal  accumulations 
were  laid  down.  An  equilibrium  was  maintained  for  some  time 
between  the  forces  tending  to  elevate  and  those  tending  to  sink  the 
land.  The  latter  prevailed  so  far  as  to  make  possible  a practically 
continuous  addition  of  littoral  deposits,  throughout  the  Rattlesnake 
and  the  Tornillo  periods.  These  beds  do  sometimes  exhibit  minor 
unconformities  without  tilting,  due  perhaps  to  the  making  and  un- 
making of  low  bars  and  islands.  They  can  hardly  have  any  other 
significance.  Volcanic  eruptions  became  more  frequent  and  also 
less  distant  in  place,  as  shown  by  the  greater  frequency  and  thick- 
ness of  occasional  ash  beds.  At  last  these  materials  became  copious 
and  largely  exceeded  the  other  ingredients  of  the  deposits  of  the 
sea.  Land  was  evidently  near,  if  not  right  here.  But  the  bottom 
of  the  sea  continued  to  sink  under  the  weight  of  the  Chisos  tuffs. 
Meanwhile  many  new  volcanic  vents  were  opened  and  they  some- 
times emptied  their  contents  under  the  waters  of  the  sea  as  weil 
as  on  the  near  lands.  New  mountain  folds  were  already  forming, 
involving  the  earlier  limestone  formations  of  the  age,  for  worn 
pebbles  of  these  are  found  in  the  Chisos  beds.  The  making  of  the 
Rocky  mountains  had  now  begun. 

At  the  end  of  the  Cretaceous  age,  or  perhaps  during  the  early 
part  of  the  Tertiary  age,  the  sea  receded  and  an  extensive  land 
rose  to  take  its  place.  There  were  volcanoes  and  mountain  folds. 
Molten  magmas  were  injected  among  the  sediments  and  poured 
out  over  their  surface.  Blocks  of  sediments  whose  dimensions  are 
measured  in  miles,  adjusted  themselves  to  orogenic  forces  into  most 
varied  positions,  and  were  folded  and  fractured  promiscuously. 


Th&  Geology  of  the  Chisos  Country. 


89 


This  mountain-making  period  continued  through  the  greater  part 
of  the  Tertiary  age.  With  the  gradual  cessation  of  volcanic  dis- 
turbances, destructive  forces  attained  to  the  ascendency  and  these 
have  since  reduced  the  formerly  much  higher  mountains  and 
plateaus  to  their  present  condition. 

K.  MINERAL  RESOURCES. 

So  far,  cinnabar  is  the  only  material  in  this  region  which  has 
proved  to  be  of  economic  importance.  The  mining  of  quicksilver 
is  now  an  established  industry  in  Brewster  county.  But  silver  and 
coal  have  also  been  found.  Attempts  at  the  mining  of  silver  have 
not  been  entirely  discouraging  and  the  mining  of  coal  on  a small 
scale,  to  supply  the  local  demand  for  fuel,  may  prove  profitable. 

1.  Quicksilver. 

It  is  proposed  to  here  briefly  state  such  general  conclusions  re- 
garding the  occurrence  of  the  quicksilver  ore  as  now  seem  to  be 
clear. 

The  Big  Bend  mine  explorations  now  (1904)  show  that  the  ore 
occurs  in  a dike  of  andesitic  rock  as  well  as  in  the  fissures  in  the 
Cretaceous  shales  and  limestones  surrounding  the  intrusive.  The 
dike  is,  in  common  with  other  such  structures  when  injected  into 
soft  sediments,  irregularly  developed,  running  out  in  a short  dis- 
tance when  followed  to  the  west  and  changing  into  a sill  above. 
It  is  a filling  in  an  irregularly  developed  fault  and  the  ore  is  in  a 
place  where  the  ground  water  or  the  mineralizing  solution  follow- 
ing the  dike  upward  would  be  impeded,  consequent  upon  a change 
of  route. 

It  will  be  remembered  from  earlier  reports  of  this  survey  that  in 
the  Marfa  and  Mariposa  mine  and  in  the  old  Terlingua  mine  much 
of  the  ore  has  come  from  brecciated  fissures  in  the  upper  surface 
of  the  Edwards  limestone.  This  was  originally  directly  covered  by 
the  next  to  impervious  Del  Rio  clay,  which  at  the  present  time  is 
mostly  removed.  Evidently  in  this  case,  also,  the  ore  was  pre- 
cipitated from  rising  solutions  at  a level  where  these  were  hemmed 
in  by  the  impervious  cover  of  the  clay.  As  cinnabar  has  been 
found  in  this  mine  also  in  a porphyry,  the  ore  is  in  that  respect  in 


90 


Bulletin  of  The  UnivOrsity  of  Texas. 


a situation  quite  similar  to  that  at  Big  Bend.  If  the  mineralized 
solution  followed  the  course  of  a body  of  an  intrusive  in  the  lime- 
stone it  would  be  likely  to  have  a comparatively  open  and  unim- 
peded passage  along  this  intrusive,  as  long  as  this  traversed  the 
limestone.  For  in  limestones  dikes  are  quite  uniform  in  their 
development.  But  reaching  the  overlying  clay  the  solution  would 
find  the  dike  less  regular  and  the  clay  also  would  hinder  its  ascen- 
sion. The  solution  would  naturally  follow  the  upper  surface  of  the 
limestone  and  precipitates  would  find  lodgment  in  its  fissures  un- 
der the  clay. 

This  theory  finds  verification  in  alomst  every  instance  of  known 
occurrence  of  ore  in  this  territory.  In  the  region  of  the  Colquitt- 
Tigne  mine,  where  the  ore  occurs  in  fissures  in  the  Boquillas  flags, - 
it  has  been  observed  that  the  veins  are  occasionally  enriched  im- 
mediately under  the  more  argillaceous  strata  which  these  flags  con- 
tain. In  Christmas  mountain,  where  cinnabar  was  discovered  last 
summer,  it  is  likewise  associated  with  a dike  of  andesite  and  is  in 
a fissure  in  the  uppermost  ledges  of  the  Lower  Cretaceous  limestone 
near  the  summit  in  the  dome.  There  can  be  no  doubt  that  this 
limestone  was  covered  by  the  more  impervious  sediments  of  the 
Middle  Cretaceous  at  the  time  the  ore  was  deposited.  Thus  the  con- 
ditions here  would  be  like  those  at  the  Marfa  and  the  Mariposa  mine 
and  the  old  Terlingua  mine.  The  rising  solutions  deposited  ore  on 
encountering  a more  impervious  overlying  formation.  Again  in 
Mariscal  mountain,  at  Lindsey’s  claim,  we  find  the  ore  at  the 
same  horizon,  and  likewise  on  the  crest  of  a fold.  At  this  place, 
as  on  Christmas  mountain,  the  Del  Bio  clay  is  absent,  but  the  base 
of  the  Boquillas  flags  is  still  left  and  ore  occurs  in  seams  between 
some  of  the  ledges  of  this  formation.  In  one  of  the  openings  made 
a small  offshoot  of  an  andesitic  dike  wras  followed  by  a thin  stringer 
of  cinnabar  on  its  upper  surface.  These  occurrences  show  that  the 
fracture  produced  by  the  offshoot  has  served  as  a conduit  of  the 
mineralized  solution  and  that  precipitation  has  occurred  at  a place 
■where  it  met  with  resistance  in  its  slow  upward  percolation  along 
the  small  “stringers”  of  the  dike  and  in  the  less  pervious  sediments 
by  which  the  stringers  of  the  dike  are  surrounded.  In  one  of  the 
openings  in  the  Boquillas  flags  on  the  north  end  of  the  mountain 
there  were  three  seams  of  cinnabar  about  one-fourth  inch  thick  and 


The  Geology  of  the  Chisos  Country. 


91 


twenty  inches  apart,  following  joints  which  cut  the  sediments  run- 
ning transversely  to  the  axis  of  the  mountain. 

Another  circumstance  points  to  the  correctness  of  the  same  view. 
This  is  the  fact  that  the  principal  ore  deposits  have  been  discovered 
on  crests  of  anticlines  or  else  on  lines  of  changing  dips,  where  the 
flexures  are  convex  upward.  The  deposits  on  Christmas  mountain 
and  on  the  north  end  of  Mariscal  mountain  are  examples  of  the 
former  instance,  and  the  ore  at  the  Marfa  and  Mariposa  mine,  that 
in  the  Chisos  mine,  and  that  on  the  east  side  of  Mariscal  mountain, 
illustrate  the  latter  mode  of  occurrence.  This  last  mentioned  oc- 
currence is  at  a point  about  three  miles  west  and  one  mile  south  of 
Solis7  ranch.  The  cinnabar  is  here,  as  elsewhere,  frequently  in 
and  near  a dike.  The  dike  in  this  instance  cuts  the  lower  edges 
of  the  Boquillas  flags. 

According  to  this  view  of  the  origin  of  the  ore,  we  shoud  expect 
the  principal  deposits  to  lie  in  places  toward  which  ascending  cur- 
rents have  converged,  as  on  the  “hips”  and  crests  of  anticlines. 
The  presence  of  cinnabar  in  the  intrusives  is  due  to  impregnation, 
which  has  introduced  the  cinnabar  as  a secondary  mineral.  We 
have  seen  that  profound  changes  have  been  effected  in  this  manner 
in  other  volcanics  of  this  region.  There  is  all  the  more  reason  to 
regard  the  quicksilver  ore  as  a secondary  deposit,  for  we  always 
find  the  rock  altered  in  a peculiar  way  in  the  vicinity  of  the  ore 
deposits.  The  exact  nature  of  this  alteration  is  not  known,  but  it 
causes  the  dikes  to  weather  with  a characteristic  bleached  yellowish 
color,  different  from  that  which  characterizes  the  same  weathered 
rock  in  its  usual  condition.  This  bleached  appearance  of  the 
igneous  rock  happens  to  be  especially  well  shown  near  the  two  min- 
eral localities  on  the  east  side  of  Mariscal  mountain.  It  is  notice- 
able in  some  places  where  cinnabar  is  not  known  to  exist.  One 
instance  of  this  sort  was  seen  at  a point  about  three  miles  west  and 
one  mile  north  of  Mailbox  tank,  at  the  east  foot  of  the  Rosillos 
mountains.  An  intrusive  here  weathers  to  a resemblance  of  the 
dike  associated  with  the  cinnabar  in  the  Christmas  mountains. 
But  an  analysis  of  the  rock  failed  to  show  the  presence  of  any  trace 
of  the  mineral.  This  analysis  is  as  below: 


92 


Bulletin  of  The  University  of  Texas. 


Analysis  of  an  altered  igneous  rock  from  the  east  side  of  the 
Rosillos  Mountains* 


Silica 

Alumina 

Ferric  oxide 

Ferrous  oxide 

Magnesia 

Lime 

Soda 

Potash 

Water  above  105-110  C. 
Water  below  105-110  C 

Carbonic  acid 

Titanium  dioxide 

Phosphorus  pentoxide  . 
Manganous  oxide 


62.00  per  cent. 
15.49  per  cent. 
6.00  per  cent. 
1.08  per  cent. 
0.10  per  cent. 
1.78  per  cent. 
5.10  per  cent. 
3.68  per  cent. 
2.95  per  cent. 
0.80  per  cent. 
0.88  per  cent. 
0.86  per  cent. 

trace. 

trace. 


In  some  parts  of  the  world  quicksilver  deposits  are  found  in 
close  association  with  hot  springs.**  While  the  Brewster 
county  ore  can  not  be  said  to  now  have  very  close  associations 
of  this  kind,  it  is  interesting  to  note  the  presence  of  several 
hot  springs  on  the  Rio  Grande  river  in  the  Carmen  range, 
only  ten  miles  distant  from  a known  occurrence  of  cinna- 
bar. Three  of  these  springs  emerge  from  near  the  zone  sepa- 
rating the  Boquillas  flags  from  the  heavy  bedded  limestones  of 
the  Lower  Cretaceous,  and  thus  demonstrate  the  fact  that  the 
more  clayey  overlying  flags  may  cause  the  ground  water  to  follow 
their  under-surface  in  seeking  a higher  outlet.  Some  of  the  fis- 
sures which  these  springs  have  followed  are  now  filled  with  ma- 
terials that  must  have  been  deposited  from  their  solutions,  and  they 
resemble  in  every  respect  the  gangue  of  some  of  the  cinnabar 
]eads  in  the  limestone  at  Terlingua,  except  that  the  ore  is  absent 
as  is  also  often  the  case  at  Terlingua.  It  consists  of  crystalline 
calcite  and  of  amorphous  carbonate  of  lime,  in  which  fragments  of 
the  country  rock  are  imbedded.  In  its  chemical  composition  it  re- 
sembles the  tufa  which  is  now  forming  around  the  hot  springs. 

*0.  H.  Palm,  analyst. 

**Becker,  Quicksilver  Deposits  of  the  Pacific  Slope,  U?  S.  Geol.  Surv., 
Monograph  XIII,  p.  52, 


Thu  Geology  of  the  Chisos  Country. 


93 


Analysis  of  Precipitations  from  hot  springs  in  the  Carmen  range 
on  Rio  Grande  river. 


Deposit  in  a fissure  of  an  Tufa  from  a hot  spring 
old  spring  near  mouth  near  the  mouth  of 
of  Tornillo  creek.  Tornillo  creek. 


Silica 19.26  per  cent.  19.00  per  cent. 

Alumina 1.00  per  sent.  4.02  per  cent. 

Ferric  oxide 1.30  per  cent.  1.60  per  cent.  ' 

Lime 42.95  per  cent.  40.60  per  cent. 

Magnesia 0.60  per  cent.  1.10  per  cent. 

Sulphur  trioxide 0.10  per  cent.  trace.. 

Carbonic  acid 32.80  per  cent.  30.16  per  cent. 


Water  [hygroscopic]  . ..  0.20  per  cent.  0.80  per  cent. 
Water  [in  combination].  1.35  per  cent.  2.00  per  cent. 


99.56  per  cent.  99.28  per  cent. 

The  knowledge  we  now  possess  of  the  cinnabar  deposits  in  Brew- 
ster county  seem  indeed  to  bear  out  the  conclusions  reached  by 
Geo.  F.  Becker  in  the  study  of  similar  deposits  elsewhere,  who 
ascribes  them  to  mineralized  and  probably  hot  rising  solutions. 
He  says:* 

“The  mineral  associations  in  which  cinnabar  is  found”  (in  dif- 
ferent parts  of  the  world)  “seem  to  'show  conclusively  that  it  has 
been  deposited  from  solutions.  A very  large  part  of  the  known 
deposits  of  cinnabar  are  extremely  similar  in  character,  a fact 
which  seems  indicative  of  a similar  origin.  It  is  certain  that  some 
of  the  deposits  are  due  to  precipitation  from  hot  volcanic  springs, 
and  it  may  fairly  be  inferred  that  many  of  them  were  found  in  this 
manner.  The  diversity  of  the  country  rocks  in  which  the  deposits 
occur  is  evidence  that  only  a part  of  them  can  have  derived  their 
metallic  contents  from  their  own  wall  rocks;  the  remainder  must 
owe  their  cinnabar  to  some  source  between  the  point  at  which  the 
waters  acquired  their  heat  and  the  surface.  Between  the  depth 
at  which  volcanic  foci  lie  and  the  surface  of  the  earth  there  must 
he  substances  of  world-wide  distribution  which  frequently  contain 
mercury  in  some  form  as  an  original  ingredient.  These  substances 


Reference  as  above,  p.  55. 


94 


Bulletin  of  The  University  of  Texas. 


are  probably  massive  rocks,  and  the  only  known  rock  of  corre- 
spondingly wide  distribution  is  granite.” 

On  this  theory  the  significance  of  the  association  of  the  quick- 
silver with  volcanic  rocks  in  this  region  is  also  quite  clear.  The 
sills  and  especially  the  dikes  traverse  the  sediments  to  great  depths 
and  are  frequently  so  placed  that  they  must  have  furnished  in  the 
past,  as  they  do  at  the  present,  the  most  direct  passage  ways  for 
underground  waters  which  traverse  the  formations  from  below  up- 
ward. Intrusive  sheets  and  dikes  cause  several  springs  and  seeps 
in  the  country  today.  Indeed,  it  was  on  the  basis  of  the  associa- 
tion of  the  igneous  rocks  with  the  Lower  Cretaceous  limestones 
that  the  director  of  the  survey,  Dr.  Wm.  B.  Phillips,  was  able  to 
predict  the  discovery  of  cinnabar  east  of  the  Chisos  three  years  ago. 

2.  Coal. 

The  existence  of  coal  in  the  Chisos  country  has  been  known  for 
some  time,  but  its  field  relations,  its  quality  and  general  develop- 
ment have  been  matters  of  uncertainty. 

From  the  stratigraphic  data  already  set  forth,  it  will  be  remem- 
bered that  the  Rattlesnake  beds  contain  the  coal. 

The  several  places  at  which  coal  has  been  observed  extend  from 
the  Rosillos  mountains  to  the  mouth  of  the  Ter  lingua  and  from  the 
west  side  of  the  Carmen  range  to  near  the  Cigar  mountain.  Be- 
low some  of  the  localities  are  enumerated: 

1.  In  the  north  side  of  a hill  on  the  south  side  of  the  road 
leading  southwest  from  Stroud’s  ranch  and  about  two  miles  from 
this  ranch,  the  face  of  this  coal  has  been  laid  bare  by  three  short 
drifts,  but  it  is  at  present  concealed.  The  seam  is  reported  to  have 
measured  eighteen  inches.  Over  the  coal  are  some  sandy  and 
bituminous  dark  silts,  and  under  it  are  first  some  three  feet  of 
clay  and  then  a black  carbonaceous  sandstone.  Some  silicified 
trunks  of  trees  were  noted  close  by  the  explorations.  The  strata 
dip  about  ten  feet  to  the  south.  The  intrusives  of  the  Rosillos 
mountains  are  not  far  distant  and  have  evidently  baked  the  coal  to 
some  extent.  Some  of  the  coal  taken  out  from  the  pits  was  left 
on  the  ground,  and  this  exhibited  a cubic  fracture,  an  unusually 
bright  luster  and  a very  compact  texture.  A few  pieces  were 
picked  up  which  had  these  characteristics  in  a superlative  degree. 


Tha  Geology  of  the  Chisos  Country. 


95 


and  in  which  no  flaws  could  be  seen,  although  they  had  been  ex- 
posed to  the  weather  clearly  for  a long  time.  These  have  the  ap- 
pearance of  jet,  and  have  great  crushing  strength.  Some  show 
very  faint  but  unmistakable  evidence  of  woody  structure  on  a 
^cylindrical  outer  surface.  A small  block,  which  was  shown  to  the 
writer  by  Mr.  Carruthers,  of  Alpine,  probably  came  from  this  place. 
It  measured  nearly  four  inches  in  diameter.  It  was  clearly  a coal 
formed  from  some  species  of  wood.  A well  known  gem  expert,  who 
examined  some  of  this  material  for  the  writer,  says  that,  if  found 
in  blocks  of  sufficient  size,  this  mineral  may  he  valuable  as  a sub- 
stitute for  jet.  It  resembles  jet  in  its  luster,  but  it  has  the  com- 
position of  anthracite,  as  is  shown  by  the  following  analyses: 

Analyses  of  cml  from  two  miles  southwest  of  StroucVs  ranch  A 

Anthracite  with  jet 
Anthracite  from  two  luster  from  two 

miles  southwest  of  miles  southwest 

Stroud’s  ranch.  of  Stroud’s  ranch. 

Moisture 2.44 1 1.47 

Ash 4.23 1 1.93 

Volatile  combustible  matter.  . 15.38 1 13.07 

Fixed  carbon 77.95 1 83.53 

— i • — 

100.001 100.00 

Sulphur 93 1 1.26 

The  jet-like  anthracite  represents  entire  tree  trunks  which 
form  a part  of  the  coal  seam.  The  coal  is  quite  different  from  the 
other  seams  in  the  region  in  having  so  small  a percentage  of  ash. 
Its  high  content  of  fixed  carbon  is  clearly  due  to  baking  by  the 
intrusives  in  the  Rosillos  mountains. 

2.  On  the  north  side  of  Tornillo  creek,  north  from  the  Grape- 
vine hills,  a small  seam  of  coal  was  uncovered  in  a surface  pit 
some  years  ago.  Peaty  and  bituminous  shales  have  been  observed 
in  several  places  in  the  west  part  of  the  Tornillo  flats. 

3.  In  the  country  immediately  west  of  the  Slickrock  mountain 
two  pits  have  been  made  in  which  coal  was  found.  In  neither 
place  the  seam  measured  quite  twenty  inches  in  thickness. 

4.  The  Kimble  pits  are  some  openings  made  on  a seam  which  is 
exposed  on  the  south  side  of  a hill  at  a point  two  miles  north  of  the 

* Analyst,  Mr.  O.  H.  Palin. 


96 


Bulletin  of  The  University  of  Texas. 


Chi sos  pen  north  of  Rough  run.  This  seam  of  coal  measures 
about  twenty  inches.  It  has  a considerable  dip  and  the  ground  is 
somewhat  broken.  It  is  associated  with  bituminous  and  sandy 
shales  or  clays.  An  analysis  of  this  coal  shows  that  it  contains: 


Moisture . 4.68 

Ash 16.60 

Volatile  combustible  matter 24.20 

Fixed  carbon 54.52 


100.00 

Sulphur 88 


5.  What  is  probably  the  same  seam  as  that  seen  in  the  Kimble 
pits  is  exposed  in  the  bottom  of  Cottonwood  creek  at  Chisos  pen, 
where  it  measures  eighteen  inches.  The  creek  follows  a line  of  dis- 
placement at  this  place  and  the  coal  also  appears  about  two  hun- 
dred yards  away  from  the  arroyo  on  the  north  side,  in  a gully, 
where  it  has  about  the  same  development.  In  a piece  of  this  coal 
which  was  more  closely  examined,  some  pitted  Tracheids  of  gym- 
nospermous  wood  could  be  discerned.  Some  very  small  grains  of 
rnarcasite  were  seen  along  some  horizontal  seams.  Above  as  well 
as*  below  this  coal  we  find  carbonaceous  and  bituminous  silts  and 
clays.  Its  composition  resembles  that  of  the  coal  in  the  Kimble 
pits,  with  somewhat  less  ash  and  somewhat  more  of  volatile  ma- 


terial, viz. : 

Moisture 6.12 

Ash 14.42 

Volatile  combustible  material 34.72 

Fixed  carbon 44.74 


100.00 

Sulphur 1.32 


6.  In  the  flats  which  lie  about  one  and  one-fourth  mile  south- 
east of  the  southeast  side  of  Maverick  mountain  there  are  some 
exposures  of  a thin  coal  seam.  One  of  these  which  appeared  in 
the  east  bank  of  a creek  showed  eight  inches  of  good  coal  with 
nearly  two  feet  of  ‘T>one”  or  coaly  shale  below.  An  analysis  of 
this  coal  is  as  below: 


Tiie>  Geology  of  the  Chisos  Country. 


97 


Moisture  . . • 6.46 

Ash 25.90 

Volatile  combustible  matter 34.88 

Fixed  carbon 32.76 


100.00 

Sulphur . 1.00 


.This  coal  contains  frequent  small  grains  or  pockets  of  a yellow 
resin,  from  the  size  of  a mustard  seed  to  that  of  a bean.  They  are 
reddish-brown  in  color,  very  irregular  in  shape,  and  usually  frac- 
tured by  weathering.  Mr.  O.  H.  Palm,  who  has  examined  them, 
finds  that  “they  easily  melt  and  burn  if  brought  near  the  flame. 
They  are  soluble  in  chloroform,  ether,  and  alkalies.  Their  melt- 
ing point  is  between  205°  and  210°  0.” 

7.  Some  coal  seams  were  noted  in  the  hills  on  the  west  side  of 
Terlingua  creek  about  three  and  a half  miles  south  of  Study  butte. 
None  of  these  seem  to  be  more  than  a foot  in  thickness,  but  the 
coal  is  associated  with  a -‘bony”  shale,  which  is  highly  bituminous. 
Grains  of  resin  are  present  in  the  shales  as  well  as  in  the  coal. 

8.  The  same  seam  of  coal  appears  again  in  the  hills  on  the 
opposite  side  of  the  creek  about  a mile  to  the  south,  where  it  dips 
at  a high  angle. 

9.  In  the  low  ridges  of  the  flats  between  Terlingua  Abaja  and 
the  mouth  of  Terlingua  creek  [about  three-fourths  of  a mile  to 
the  south  and  a little  west  of  the  village]  a coal  seam  crops  out, 
which  is  eighteen  inches  thick.  It  dips  30°  to  the  southwest. 
This  coal  is  associated  with  some  sandstone  ledges  and  it  is  about 
in  the  same  horizon  as  the  Chisos  pen  coal.  A piece  taken  from 
the  weathered  face  of  this  outcrop  contained  :* 


Moisture 9.10 

Ash 21.50 

Volatile  combustible  matter 37.38 

Fixed  carbon 32.02 


100.00 

Sulphur 0.90 


*0.  H.  Palm,  analyst. 


98 


Bulletin  of  The  University  of  Texas. 


10.  On  the  north  side  of  the  Rio  Grande,  about  seven  miles 
west  of  Mariscal  mountain,  the  Rattlesnake  beds  contain  some 
bituminous  clays,  that  have  thin  seams  of  coal. 

11.  Apparently  trustworthy  reports  say  that  coal  has  been  seen 
in  some  of  the  arroyos  north  of  Talley’s  ranch  and  southwest  of  the 
north  end  of  Mariscal  mountain  and  also  in  the  country  northwest 
of  Banta  Shutup.  In  both  of  these  localities  there  are  extensive 
outcrops  of  the  Rattlesnake  beds. 

The  writer  believes  that  the  Rattlesnake  beds  and  their  included 
coals  are  the  near  equivalents  of  the  coal-bearing  beds  at  Eagle 
Pass.  Both  overlie  the  marine  clays  of  the  Middle  Cretaceous, 
the  Upson  clays  and  Terlingua  beds.  The  coals  are  somewhat 
alike  in  composition  and  they  both  contain  small  grains  of  resin. 
Some  very  characteristic  impressions  of  plants  are  common  to  both 
formations,  as  shown  on  a previous  page,  and  some  fossils  are 
common  to  both.  Still  there  is  a difference  in  the  fauna  as  well 
as  in  the  nature  of  the  sediments  themselves.  The  latter  are 
somewhat  more  variable  in  the  Chisos  country,  where  the  sand- 
stones are  slightly  coarser  and  have  a more  open  texture.  The 
sandstones  are  here  also  less  persistent  when  traced  horizontally. 
All  this  indicates  less  open  waters  in  which  sedimentation  took 
place  in  the  Chisos  region.  This  view  is  corroborated  also  by  the 
difference  in  the  two  faunas.  At  Eagle  Pass  ammonoids  are  quite 
common,  while  in  the  Chisos  country  these  are  rare  and  the  oyster 
family  is  much  more  frequently  in  evidence.  This  has  a bearing 
on  the  economic  features  of  the  coal  seams.  We  should  not  ex- 
pect to  find  these  as  constant  in  the  more  variable  sediments  in  the 
Chisos  country  as  they  have  proved  to  be  in  the  Eagle  Pass  region. 
They  were  made  nearer  to  the  shore  in  the  western  country  and 
will  naturally  be  more  variable  and  uncertain.  On  the  other  hand, 
it  has  been  shown  that  the  Rattlesnake  beds  belong  to  the  true 
coal-bearing  horizon  of  the  Upper  Cretaceous  series  in  the  West, 
and  that  they  will  bear  exploration.  They  outcrop  in  a circular 
belt  around  the  Chisos  mountains,  and  this  belt  can  not  yet  be 
regarded  as  sufficiently  explored.  It  is  true  that  all  the  outcrops 
which  have  been  examined,  show  only  thin  seams.  The  variability 
of  the  beds  proves  that  local  and  sudden  changes  are  to  be  expected. 
Such  variations  may  be  for  the  better  as  well  as  for  the  worse. 


The.  Geology  of  the  Chisos  Country. 


99 


At  any  rate,  some  thin  seams  are  already  known,  and  in  a country 
where  fuel  is  as  scarce  as  here,  some  of  these  might  prove  profitable 
to  work  on  a small  scale  under  present  conditions. 

3.  Silver. 

On  the  north  side  of  Altuda  mountain  some  silver-bearing  galena 
has  been  mined.  The  workings  are  on  both  sides  of  an  arroyo, 
which  comes  from  the  west  and  runs  on  the  north  side  of  the  moun- 
tain. The  workings  consist  of  some  three  hundred  feet  of  drifts 
and  shafts,  some  of  which  follow  the  contact  of  the  ore-bearing 
rock  and  the  Cibolo  limestone,  which  forms  the  country  rock.  On 
the  south  side  of  the  arroyo  there  rises  from  this  limestone  a 
wedge-shaped  peak  of  partly  silicified  material,  which  clearly  at 
some  time  was  a filling  in  a cavern  in  the  limestone  above  which 
it  now  rises.  This  peak  consists  of  an  altered  conglomerate  or 
breccia,  containing  pebbles,  boulders,  and  large  blocks  of  sandstone, 
of  various  kinds  of  igneous  rocks,  of  granite,  and  of  limestone 
from  the  Cretaceous  as  well  as  from  the  Carboniferous  strata. 
The  infiltration  of  siliceous  material  has  rendered  this  mass  some- 
what resistant  to  the  agencies  of  weathering  and  erosion,  and  so 
the  country  rock  has  suffered  more  rapid  removal  by  these  agencies 
while  the  more  enduring  filling  remains  in  part  intact.  Most  of 
the  ore  occurred  in  irregular  leads  near  the  contact  of  this  filling 
with  the  limestone  in  the  wall  of  the  ancient  cavern.  On  the 
north  side  of  the  arroyo  a siliceous  tufa,  probably  a precipitate 
from  underground  hot  solutions,  is  largely  in  evidence  over  a part 
of  the  surface  of  a large  hill.  Its  contact  with  the  limestone  can 
be  traced  in  several  places,  and  occasionally  it  takes  on  the  ap- 
pearance of  an  igneous  intrusive.  Some  very  rich  ore  has  been 
taken  out  from  one  of  the  explorations  near  the  contact  at  this 
deposit  with  the  limestone. 

In  the  mind  of  the  writer  there  is  no  doubt  that  the  mineral 
deposits  at  this  place,  whatever  their  value,  have  essentially  the 
same  origin  and  history  as  the  lodes  in  the  Shatter  mine  near  the 
Chinati  mountains.*  The  material  filling  the  ancient  caverns  is 

*See  the  Geology  of  the  Shatter  Silver  Mine  District,  Presidio  county, 
Texas,  Bulletin  The  University  of  Texas  Mineral  Survey,  No.  8,  p.  54  et  seq. 


100 


Bulletin  of  The  University  of  Texas. 


much  more  extensive  here  at  Altuda,  and  it  possibly  consists  in 
part  of  intrusive  material.  What  bearing  this  may  have  on  the 
quantity  of  ore  no  one  can  say.  Lodes  vary  from  barrenness  to 
the  richest  deposits  known.  They  are  always  expensive  to  explore. 
The  explorations  at  Altuda  are  as  yet  not  extensive  enough  to 
give  an  estimate  of  the  quantity  of  the  ore,  which  in  a pocket  on 
one  of  the  drifts  was  very  rich. 

4.  Phosphate. 

Some  of  the  rock  samples  taken  from  the  strata  of  the  Trenton 
age  near  Eidge  spring,  south  of  Pena  Colorado,  were  found  to  have 
specimens  of  the  small  gasteropod  Cyclora.  This  fossil  is  common 
in  the  phosphate-bearing  ledges  of  the  Trenton  horizon  in  Ten- 
nessee, and  its  existence  here  suggested  the  desirability  of  having 
aii  analysis  made  in  order  to  see  if  the  limestone  would  contain 
phosphate  in  this  western  field.  Two  small  specimens  of  the  lot 
brought  from  the  field  were  analyzed  and  one  yielded  6.03  and  the 
other  1.70  per  cent  of  phosphorus  pentoxide.  This  is  clearly  in 
excess  of  the  usual  phosphate  content  of  limestones,  and  suggests 
the  possibility  of  workable  phosphate  rock.  The  author  had  no 
suspicion  of  the  existence  of  any  phosphate  in  these  rocks  at  the 
time  he  was  in  the  field,  and  the  specimens  analyzed  were  collected 
as  types  of  the  rock  and  taken  for  the  fossils  they  contained.  The 
regularity  and  persistence  of  the  bedding  of  the  Palaeozoic 
sediments  will  render  exploration  for  phosphate-bearing  rock  easy, 
and  should  any  strata  prove  to  he  of  sufficiently  high  grade  there 
is  little  doubt  that  they  will  be  extensive.  The  field  that  should  be 
explored  is  coextensive  with  the  Caballos  ridges  between  Warwicks 
and  Del  Norte  gap  and  for  some  distance  to  the  northeast  of  War- 
wicks. In  appearance  the  phosphate-bearing  rocks  do  not  differ 
much  from  other  limestone. 

5.  Brick  Materials. 

The  reduction  of  the  quicksilver  in  the  Terlingua  mines  has 
created  an  unsteady  market  for  brick.  This  market  has  been  sup- 
plied by  Mr.  John  Dryden,  who  finds  a suitable  clay  on  lower 
Terlingua  creek.  His  clay  consists  of  the  upper  three  or  four  feet 


The,  Geology  of  the  Chisos  Country. 


101 


of  wash,  or  alluvium,  near  this  stream.  The  clay  is  tempered  by 
the  addition  of  a “sand,”  which  is  really  the  soil  formed  from  a 
thoroughly  disintegrated  intrusive  andesitic  rock.  The  brick  is 
of  excellent  quality  and  has  proved  to  stand  a white  heat  for  sixty 
hours.  It  is  free  from  cracks  and  has  a dirty  gray  or  cream  color. 
It  is  hand-made,  sun-dried,  and  burnt  in  square  up-draught  kilns 
with  thick  adobe  walls.  The  total  output  up  to  this  time  (1904) 
is  1,250,000  bricks.  The  brickyard  and  kilns  are  located  a short 
distance  north  of  Dryden’s  ranch. 


BULLETIN 

OF  THE 

UNIVERSITY  OF  TEXAS 


SCIENTIFIC  SERIES 

VOLUME  ONE 


AUSTIN,  TEXAS 


1905-1907 


CONTENTS 


Bulletins  1,  2,  3 and  4 are  reprints  properly  belonging  in  the 
Reprint  Series , and  are  therefore  not  to  be  bound  with  this  volume. 

5.  Test  of  a Vertical  Triple  Expansion  High-Duty  Pumping 

Engine  in  Operation  at  the  Water  Works,  San  Antonio, 
Texas,  by  A.  C.  Scott.  52  p.,  illus.  pi.  1905. 

6.  Vegetation  in  the  Sotol  Country  in  Texas,  by  W.  L.  Bray 

24  p.,  pi.  1905. 

7.  Observations  on  the  Habits  of  Some  Solitary  Wasps  of  Texas, 

by  Carl  Hartman.  72  p.,  pi.  1905. 

8.  The  Protection  of  Our  Native  Birds,  by  T.  H.  Montgomery, 

Jr.  30  p.  1906. 

9.  The  Austin  Electric  Railway  System,  by  members  of  the 

senior  class  in  Electrical  Engineering,  1906.  123  p., 

illus.  pi.  1906. 

10.  Distribution  and  Adaptation  of  the  Vegetation  of  Texas,  by 

W.  L.  Bray.  108  p.,  pi.  map.  1906. 

11.  A Sketch  of  the  Geology  of  the  Chisos  Country,  by  J.  A. 

Udden.  101  p.  1907. 


